I can think of a few reasons. Drivers should be protected from a mishmash of locality-specific traffic configurations and rules, so it's beneficial to have the state enforce some uniformity and best practices (and obviously a good bit of that is derived from the Feds).

Also, the state (and thus the state's taxpayers) ends up paying for a good chunk of local roads. Cambridge or any other locality shouldn't have free reign to let NIMBYism make life difficult for out-of-town commuters.