1. Layering ~ a quick drying time allows you to add layers of paint on top of each other without having to wait around a long time and watch paint dry ;). That being said, if you need more drying time there are additives like Golden's Open Medium that serve that purpose. Layering acrylics also give you the ability to use some of my favorite techniques such as etching into the wet paint to reveal the color beneath, or cover up large areas without creating mud.
2. Mixes well with other media ~ acrylics are very medium-friendly :) You can layer them on top of watercolors, paint pens, water-soluble crayons, pens, etc.. You can also go over top of them with oil sticks and paint pens - just be sure to let the paint completely dry first or you'll quickly ruin your paint pen! They also work well with collage - I use a gel matte medium to adhere the collage bits on top - this you can do even when the acrylics are wet.
3. Translucency ~ Many of the acrylics have a translucent quality and with these you can see the the layers below. This can add a great richness to your painting.
5. Easy to work on paper ~ When I'm working on bigger pieces I keep a stack of 140 lb watercolor paper nearby. I use it clean off my brushes, take a break from the big painting or just to use up remaining paint at the end of a session. I love this practice because the pieces are small and no pressure attached to them - because it's just paper and it holds the paint really well. If I don't end up with a composition I like I will slice & dice them into bookmarks! These make great extras to give to people who buy my work and, of course, I put a sticker with my info on the back :)