Here are a few reasons:
It's easy. It's fun.
It makes your data useful to science in a broad array of ways - consider the recently-completed 5 years of field-work on the Wisconsin Breeding Bird Atlas II, which gathered data from
125,641 checklists, submitted by 2,147 participants
Here's a great set of ways to improve your checklists:
https://support.ebird.org/en/support/solutions/articles/48000795623-ebird-best-practices
