Why is it considered rude to arrive on time when it comes to social functions?

It isn't rude to show up on time, but some people feel that arriving 10 to 15 minutes late is a courtesy that allows the host(ess) to finish up any last minute details. Any later than that starts to fall into the rude zone. Showing up early is sometimes considered rude too, as it catches people unprepared. If you are wondering if they might need some assistance setting up, it is always better to call and ask first.