Since late 2015, starting with a Christmas set, I've been fortunate to collaborate with several groups of writers. The books have featured themes including Christmas and Pets, Valentine's Day and Pets, Paranormal Romance and Friends-to-Lovers.
Although this form of publication is relatively new to me, it has been around for a while. I'm coming in on the tail end in terms of popularity, but there are some great advantages to being in a boxed set.

1. Expanding your skill set. I've been pleasantly surprised to find that I like writing sweet romances and have a knack for it. After learning from others, I organized the boxed set that is pictured here.
2. Working within certain word counts. Most of my books are over 60,000 words, so confining myself to between 20k-40k words was a new challenge, which I completed successfully.
3. Learning new marketing and promotion tactics. In each set, every writer is responsible for some form of promotion/marketing. I've become better at making teasers and coming up with ideas for visibility.
4. Exposure to new readers. Some readers like the value they get from buying a bundle of books at a great price. The pay off for writers is being discovered by new readers.
5. Boosted Amazon Author rating and visibility. With 8-12 persons promoting a set, there's a greater reach than with an author promoting a single book. Two of the sets I mentioned reached #1 bestseller status, which naturally boosted my ranking on Amazon and which author doesn't need that?
6. Greater body of work for writers. Since I've been writing novellas, I've completed five of them, three of which have been published. One will be out in June and another will be released next year.
Of course, there are disadvantages to being in a boxed set, chief of which is that the earnings have to be split based on the number of writers. However, if you aren't making the same amount as a single author, then this isn't a major problem. Also, not everybody will do the same amount of work, but with most pulling in the same direction, things do work well.
Through working on boxed sets, I've met a host of wonderful writers who have been generous with their time and skill with graphics, promotion and marketing savvy. What say you? Is this something you think you'd participate in? Why or why not?