Submitting your timesheet is optional in the program. But if your organization requires it, this is what you're looking for. Learn how timesheets can be submitted and approved.
The biggest reason for employees to submit timesheets is that you get another set of eyes on the hours. Everybody makes mistakes. This way, a manager can quickly look over the timesheet to make sure hours are logged for the correct projects and clients. You get one more chance to check them out before submitting invoices that cannot be changed. After all, you can't go back to the client and and say, oops. You have to get it right the first time.
Another advantage is that you can lock timesheets after reviewing them. Locking means employees cannot change them. It's important to lock down timesheets before reviewing so no changes can accidently come in during the review process. That would mess up everthing. Sometimes employee enter hours into the wrong week or month, and locking prevents that. It prevents situations where time is accidently entered into a date range that has already been billed. That time is effectively lost. You can't go back to previously billed periods and demand more, just because somebody added some hours. It's better to add those hours into a new period that has not yet been billed.
There are other reasons to approve timesheets, but these are the biggies.