So I'm making these fancy maps for the city kit and it takes a long time and so naturally I'm asking myself why I'm bothering to do it.
After all, traditional grid maps are clear, easy to make, familiar to all DMs, and automatically include all the information a party expects to have. (For example--in the batcave map, how wide is the batmobile ramp? Can't tell. And, for that matter, how do you even get from Wayne manor to the cave? Can't tell. It's hard to squeeze all that boring stuff in to a picture.)
So, why bother? 2 main reasons:
#1 Well-executed picture maps are--to a DM--really fucking useful.
Traditional map: Let's say the PCs are in room 14. Cool. You look down and find the little "14" and you know all about room 14.
However, let's say somebody starts listening at the doors...what's the north door sound like?--now you've got to look up room 18. West door? Room 31. South door?...chase down room 356. You're flipping and flipping.
Or say you're trying to decide when to break for lunch...well how close are the PCs to anything interesting? Stop now or on a cliffhanger? Or what if somebody starts detecting magic--how close is the nearest magic?
With a picture map, you not only know that room 14 contains no giant gorilla, you know that room 17 does and it's close enough to smell that feast the wizard just summoned.
So, a map that makes you go "Ohhhh, that room" as soon as you look at it is gonna be helpful. A picture map doesn't have to have a picture of everything that's in the room, it just have to make each room distinctive enough that it triggers the DMs memory.
It's not such a big deal if it's a dungeon you wrote yourself and so you remember it all, but if it's a module and so it's something somebody else wrote, you're likely to appreciate reminders about what exactly is where.
Reason #2: In a product, picture maps are efficient. Most RPG stuff has pictures. Most D&D stuff has maps. Combine them into one thing you save money, you save space, and best of all the person buying it can get a better idea of what kind of thing they're buying than if they flip through it (or the pdf preview of it) and see an image which simultaneously shows the structure of the adventure and the kinds of things that show up in it.