Only 2 things i can think of (of the top of my head)
You can be sure that the application wasn’t tampered (and does exactly what the source defines) without having to relying on signing/encryption by any other parties.
You might be able to increase performance or just ensure correct functionality by tweaking somethings (compile arguments, settings) Might be userful if you have special hardware/system, which isn’t supported otherwise.
Only 2 things i can think of (of the top of my head)