The global rules for cost commitment management in D365 Finance is that this cost is raised for a significative time before the real cost (ex: when purchase order posted, when hours costs entered in journal, when expense entered in expense report,.....). So normally, the committed cost is usefull when it is identified at the beginning of the process, to be able to identify and report this cost.

For the Project Timesheet, the committed cost is raised when the timesheet is approved, that mean at the end of the process, just before the timesheet is posted (some minutes before if you use the automatic posting that is 99% the case). So this commited cost feature can never be used, because it is not at all a commited cost and is never used in companies. It is a pity not to have a normal commitment feature in this case, as it is an important process in the companies.