If you're working for a government grant or project, you are required to fill in timesheets. I knew people in Lockheed, Grumman, etc. who had to diligently fill out which project they were working on, so that their time could be billed to the right grant or agency.
While not quite a legal requirement, at the last company I worked for the timesheet was described as something like: If we don't give the government timesheets, they don't pay us our salaries.
The one before that said the timesheets were used to do reporting for R&D grants from the government.
I've worked at companies where accounting was needed to qualify for R&D tax credits, which required tracking what percentage of developers' time was spent on particular projects. My bosses at those times said something like "we have to fill these out or the company will pay more taxes, they're not for me. Make them as accurate as you can, but don't obsess over them. I'm not going use them to manage your time."
In Australia some businesses can get tax deductions of 140% for R&D time. While subcontracting to a software firm I used to fill them in for this exact reason.
