Last year, I participated in a reading challenge called Reading Outside the Box. The year before that, I did an Authors A to Z reading challenge. This year, I'm participating in a more generic challenge on Goodreads to read at least 100 books in 2015.
Why do I partake in these reading challenges? First of all, I believe that every writer should read. Anyone who thinks they can be a
good writer without reading books is delusional IMHO. That's like
saying that you can be a chef without ever tasting food. It may not be
impossible, but it's very, very, very difficult.
The additional benefit of reading challenges is that it forces me to read outside of my comfort zone. If all I read were YA books, I'd think that all stories involved insta-love and love triangles, and that's probably what I'd write (or not, since I despise insta-love and love triangles). With the Reading Outside the Box and Authors A to Z challenges, I had to read books that I normally don't. Some were not my cup of tea, as expected, but others were surprisingly good and I found myself learning from those books. Reading them could only make me better as a writer.
The thrill of finishing a reading challenge also makes me feel good. Who doesn't like completing a challenge with some degree of difficulty?
Finally, I do reading challenges because... reading is fun! Even if you're not an author, the last two points make reading challenges worthwhile.