The key difference between POST and PUT & DELETE is idempotency. To quote wikipedia "This is a very useful property in many situations, as it means that an operation can be repeated or retried as often as necessary without causing unintended effects. With non-idempotent operations, the algorithm may have to keep track of whether the operation was already performed or not."

Read Fielding's dissertation please, or any distributed systems text on the basics of RPC. HTTP is for more than just CRUD websites.