protectorates are just...so you feel good about yourself, I guess. Nothing much. Protectorates aren't much use unless you happen to be the protectorate, which usually means that faction won't attack you unless you attack them, and you get a big big big injection of money into your economy to boot. Sweet. Accept becoming a protectorate if someone demands you do so in the start of the game; it will help you to no end, unless you already have zero cashflow problems, i.e. unless your name is Greece, Macedon, the Seleucids, or Egypt.

And remember to immediately cancel military access so he can't march past your forts or trample over your newly-sown wheatfields unless he's invading, just so you know.