My Black Base Philosophy
You've spent hours painting your new badass archmage that your players will be facing at the next game. But what is he standing on? Many painters don't consider a miniature complete until it's been based in some manner. Some people just paint the top green or brown or leave it black, while others take it to a whole different level.
I like to keep my bases matte black and featureless 90% of the time. The most obvious reason why is that it simply takes less time. If I want to ambush a party of adventurers with 10 goblins in 5 days I don't want to spend the extra hours for PVA glue to dry. It reduces the painting load as well. So this is great if you're kind of lazy like I am. But that isn't the primary reason I do it in all honesty.
For display pieces or miniatures intended for a painting competition, doing a detailed base places the figure in a context and helps lend some character to the mini itself and should probably be considered mandatory in those cases. For the average painter who is readying their figures for the game table they have a choice to make about how far they want to go with basing.
The same way that a really cool detailed base lends context to a display piece, using a featureless black base makes the miniature void of context. Zombies can show up in a lot of places in Dungeons and Dragons. If I base the zombie with grass what happens when it shows up in a sewer? If I base it with dirt what happens when it attacks the players in the wizards mansion?
Roleplaying games are a story taking place collectively in everyone at the tables heads. I take the stance that everything on the table should serve to help fuel the imaginations of the players. By not putting a lot of frills you highlight the character or monster so the player can drop it into their imaginary scene without having to do any mental editing.
An adventure could go to any number of locations and even as the DM I can't always know exactly when and where a specific fight may take place based on the choices of my players and I want my minis to fit in anywhere. I do make exceptions, of course, for bosses or NPCs who I know will be in a specific location. In that case the context of the base detail is another storytelling tool. For instance, the barkeep miniature I put on a nice wood grain floor like you might find in any tavern across Faerun.
Wargames on the other hand are expected to be represented explicitly rather than in the imagination. Games like Bolt Action and Warhammer 40k are usually played on highly detailed 3d terrain where every aspect of the battlefield is physically on the table. For those kinds of games I take a middle ground with a simple dirt with splotches of grass as a kind of all-terrain compromise.
If you're putting together minis for role playing though? Stick to the black bases is my advice. It is less work for you and a better experience for your players.