I have to do timesheets as well, although in the two years I work here, I may have had 30 client billable hours.

Our department hardly makes money from clients directly, yet we maintain the most profitable software our company has. But we get "hired" to do work (typically, on a project bases) for other departments. Projects which can take a few hours to a few years. We use time sheets to keep track of how many hours we spend on a project. This information is used for several things. One of the important things is to determine how much money other departments have to pay us. And it's used to measure how well our estimates on how long the project would take were. There's also global analysis done, to see which products take more to maintain/develop. AFAIK, the numbers aren't used for tracking individual prestations - and it's certainly not used for performance bonusses (our performance bonusses are mostly based on how the company is doing as a whole, with only a small variation in individual bonusses - you've got to be really remarkable (either way) to get more or less than the average)

I hate filling out the timesheets, but I do recognize their value for the company. We can use up to 15 minute intervals, but I usually estimate how many hours I've spend on a project on an entire week, and average that out over the days. But that's easy as I usually only work on one project. Projects are typically subdivided in subcategories (knowledge gathering, development, testing, documentation, delivery).