Time registration can be a tedious task in a stressful working environment, but with limited project lists it will be much quicker. Simplifying is the key!
When working with multiple projects and users you do not want to overwhelm your users with information they do not need or endless drop-down lists. With this option of limitation you allow your users to only see the projects that they actually will be working with.
The advantage for the users is quicker and easier time registration - a win-win option for all.