it makes your workplace more attractive to developers they might want to hire, it shows it's a fun place to work
it's also a way to try ideas out and have developers on your company start collaborating that might not be on the same team
software developed might get additional help from the outside through PRs.. that might lead to finding people you want to recruit or work with for freelance gigs..
In short:
it makes you look good to help finding talent
promotes more cross-collaboration across teams in your company
the software developed might benefit both your company and others, and you get external help to make that software even better
it might make it a more fun place to work, especially if devs are allowed to spend x hours on it per week, so it might help to retain staff.
I really love solving fun and interesting problems with software. I am a full-time software developer with over a decade of experience doing a bit of everything…Android, web, backend, scripting, etc.
In short:
Thanks!