Three reasons I think colouring books are fab:
Colouring is good training
When I started learning piano, I used to play awful repetative tunes - Suzuki method (Twinkle Twinkle Little Star in five variations!), Hanon exercises - that drove the other people in the house nuts. But I didn't know how to play piano and the exercises were very useful, they got me used to the idea of sitting down and focusing on playing something, and taught my fingers to behave themselves. Now when I hear those tunes woven into music mixes, it's kind of heartwarming: those were some of the building blocks that made my brain how it is now. Not everyone can sit down in front of a blank piece of paper and fill it with images and there's nothing wrong with having some structure. I spent a good deal of time copying comics, not adding anything of my own to them, and it taught me the comics' rhythm and got my hand used to making the marks I wanted to make. People learn to play piano as adults; there's no reason they shouldn't learn to colour, too.
Colouring is hard
Colouring can be therapeutic but it can be teeth-grindingly difficult, too. When I make comics, often the colouring takes me longer than any of the other steps. To colour something well, my brain has to make lots of quick decisions, sometimes intuitive and other times carefully researched. Finding colour palettes we really love and can use in our work is a holy grail for illustrators. Some of them never really find a palette or palettes and skitter about using too many colours, or colours they don't feel are as good as they could be. I've done some complicated studies, trying to teach my brain how to place colours around an image so they're balanced in a way that makes the image communicate something as effectively as possible. There's a lot of science behind how colours work (cool colours vs warm colours, colour tonality, how different colours evoke different emotions, how colour moves the eye around in a picture and make objects pop and recede). I still often get thing wrong.
Being a colourist can be a whole profession in itself, and people who are good at it are beloved of the comics people who hire them; a top professional comics colourist can make all the difference between a comic that makes you want to read it and one that repels you so you don't even bother settling down to look at the text or line art. Don't underestimate the work behind top-quality colouring, it can be at least as complicated as Sudoku! Sometimes much harder.
Colouring is fun
Some of my favourite childhood memories are sitting together colouring with other people. It's like putting together a jigsaw puzzle, a great way to give hands something to do while people spend time together. It's fun to fill a well-drawn image with colour, and in doing so, people get inside that picture and understand better how the drawing works. You really LOOK at someone's picture when you sit down and mess around with it. For that reason, well-drawn colouring books are more fun (and better training) than badly drawn ones; not all colouring books are equal. There's some great stuff out there right now, better than when I was a kid.
If you don't like colouring books, don't buy them, but I'm glad to see more people putting drawing materials to paper!
AND... if you've read this far, here's a TOP TIP: I hit on a great trick when I used to enter colouring competitions when I was a kid: PATCHWORK. If the other kids filled a space with solid colour and I filled it with a patchwork design (including the little stitches between colour blocks), I always won. I got some good swag! It struck just the right crafty home-y note with middle-aged judges, so there you go. TOP TIP.
Edit: I love this poem, Colouring In by George Kirk.