It helps to limit spammers who would like nothing less than to post spam much more quickly than every 30 minutes. Easier to clean up one spam post than 10.
Also some people just don't quite get the site when they start out. They answer off-topic questions or don't answer properly at all or ask questions in the answer space or add me too as an answer. We'd like to be able to catch and warn such people before they become answer banned entirely.