have always thought timesheets are stupid, and are mostly only done 'because we've always done it' and people think the sky would fall in if you didn't. Here are some reasons why they're stupid:

• generally, you have to fill in details of leave taken and public holidays, etc. Don't you already KNOW when public holidays are? Why do I have to tell you on a timesheet? I already completed a leave application, which was approved, WHY do I need to tell you AGAIN in the timesheet? You have to do these things because they don't have a system that can tell them what they already (should) know.
• you know what my role is (eg, developer, tester, business analyst, etc) and you know what project I'm working on (don't you? If you don't know what project I'm working on, then isn't the company/management COMPLETELY clueless?). So, if for example, I'm a developer working on Project X, why do I need to tell you on my timesheet that this week I spent 8 hours coding for project X? Some organisations are even stupider, and want you to record time at a task level (as distinct from a project level), which always just results in people giving you rubbish info anyway - because, in any 1 hour say, they spent some time answering emails, they helped somebody out with 'X', they spent a few mins getting coffee, or whatever, etc, etc. So, usually, people can't remember exactly how much time they spent on task 'X' anyway, so they just make shit up. And they know it's stupid, so they hate doing it, so it just pisses people off.