When we say посылать, we look at a situation from the position of a sender. Посылать means all the actions a sender performs to send something. But посылать says nothing if a thing was actually received or not.
Он послал другу письмо. — He sent a letter to his friend.
When we say присылать, we look at a situation from the position of a person who receive something.
Он приcлал другу письмо. — He sent a letter to his friend and the letter was received.
Both verb are used widely. The choice of the verb depends solely on what position you stand at: talking about a sender or talking about an addressee.