Basically, people like it because they can pretty much decide themselves what they want to see on their dash, because people make Tumblr blogs for EVERYTHING. Want to see dessert recipes? Gotcha. Want to learn why some cute videos with animals are actually bad? You got it. Want to idefk, watch a lot of weird porn based primarily on Sesame Street? You'll probably find that.
The only problem can be starting off without following any blogs, and trying to find blogs to follow. After that, you're good to go.
It's gotten kind of a nasty reputation lately though, what with how many weird people are there.