How do you use tables when playing D&D?
At first, the answer seems obvious; when you want some, roll on the table. However I realize, upon reflection, that the tendency for look for answers from without, rather than within, is not always an ingrained habit. Be honest - how often do you double check yourself against a reference book? You're pretty damn sure Cal Ripken was the shortstop for Baltimore in 1991 - he won his second MVP that year - so you don't bother to look that up. Why would you? But how many facts are you that sure of in your daily life that aren't just pulled out of your ass when you're questioned?
A table in D&D is a fact you don't know yet.
That seems absurd. Tables don't give facts, they give results - and the results are based on the list you incorporate into the table when you create the thing. True enough - but here's the kicker: when you're describing what beast stumbles out of the woods ahead of the party, and you turn to a table to determine the beast, once the result comes up it is a "fact." It is, if you accept the table as law.
Thus the question, When do you use a table in D&D?
Seriously. When do you? Because the tendency is to decide yourself as DM what's the best monster to appear at that particular time, given the pattern of the campaign up to that moment, and fuck the fucking table. The table doesn't have a sense of drama or idealism (what would be absolutely perfect). The table doesn't care that the party is better prepared to fight fire elementals than earth elementals. The table can't create a fabulous set of campaign dominoes all designed to tip and fall in just the perfect sequence. The table is a cacophony. It is a sour note. It is an unwanted cat in heat at a Haydn recital.
So why use a table. Why restrict your DM genius to any set of references that can only get in the way of your conducting the game? Screw what the map says about this hex; screw that we're in a northern climate, screw the normal habits of the gods that make this move unlikely, screw what elves do or what dwarves hold sacred or how fast horses can run. I'm RUNNING A GAME HERE ... screw the inconvenient details.
I think more games climb onto rails as a result of the DM having this or that concept about "what would be perfect right now" than because the DM has determined the fate for every person in the party. It's an easy thing to do. There isn't a DM in the world who hasn't had a moment of inspiration between runnings along the lines of, "Wouldn't it be fantastic if ..." And boom - the game is on rails.
It's hard to resist a tendency like that ... and in the big picture, I don't think it should be resisted. The human imagination is wide enough to have a really, really good idea once in awhile. Sometimes a good idea is worth plonking into a campaign, randomness be damned.
Still, if you are plonking EVERY idea into a campaign, without any outside influence, then not only are you forcing the party onto a set of rails, you're forcing yourself, too. Please allow me to explain.
You, O Gentle Reader, who are a DM, are a composite of your experiences ... and as such, you are a miasma of conflicting beliefs, prejudices, flat-earth ideals and habits, all of which combine to resist new ideas, no matter what the source. If you are self-satisfied enough, you begin to despise new ideas, particularly where they challenge your old ideas, and thus the comfort zone you've built up all these years. Yet Art, my dear reader, is not created from a comfort zone. Art demands the injection of new perspectives, and most certainly the blowing apart of things like "perfection" or "what feels right" for this particular moment. If you spend too much time shaping your world in the manner that best suits you, you will only create the reflection of all your limitations ... and worse, those limitations will calcify, and your static world will atrophy and die.
An elaborate table, with lots of results which may seem like a moment of cacophony, can serve as the gemination of new, as yet unthought of ideas. The monster your generate randomly may SEEM like a bad idea - it may not seem like the wrong thing to throw at the party at this given moment ... but in fact, it may also force you to CREATE a justification or a purpose for it to be there logically. That might be a creation you would never have conceived of in your the habitual manner of your campaign. The table forces you to think outside your own box ... and may establish a precendent for future activities that otherwise you might never have considered.
In other words, the table doesn't obey your limitations; and by obeying the table, you must cast aside your limitations and improve as a DM.
So when should you use a table? As often as freaking possible.
You must learn to second guess yourself, all the time. When you think you have the "perfect" answer, ask yourself with real purpose, just what the hell does "perfect" mean? Convenient? Predictable? Ordered? Perhaps what is needed here is something that is not perfect, but something that is wild and uncontrolled, something that will tear down the flat, featureless walls of your game and push you into corners of activity where you will find yourself wallowing. If you're uncertain of where a game might take you, imagine how wide open and undisciplined your world might seem to your players! They might be able to predict YOU ... they will never be able to predict your tables.
The obvious step from here is to create tables for everything and anything that might not be clearcut. Why not a table that defines the hazards a road might offer? Why not a table that challenges a player to overcome some obstacle in purchasing a horse? Why not tables for the residents of an Inn's common room? Why not a hundred tables, so that when you feel your world needs some shaking up, you can screw yourself and get the "facts" from something outside your rule?
A little NOISE might do your campaign a lot of good.