To Christians: Why did Paul need to write letters?

In discussions of why Paul leaves so many details about Jesus out of his writings, telling us nothing about what Jesus did during his life and relating no events until after his resurrection, it's often said that Paul didn't *need* to tell people these things, because he was writing to Christians who already knew them. While this answer is unsatisfactory for several reasons, it does raise an interesting point if true:

If oral tradition was so successful at relating information to Christians that Paul didn't need to tell them about Jesus, *why did he have to write them letters at all?*

Apparently, oral tradition was enough to answer questions by Christians about whether Jesus had turned water into wine in Galilee or in Jerusalem, or about who exactly the 12 disciples were, or about what Jesus had preached about divorce. Yet the moment we turn to Christian questions about church practices, suddenly these channels are insufficient, and Paul needs to write letters. And when they stop asking these questions, and instead start to ask questions about what Jesus had told people to pray and whether or not Jesus had preached about giving tithes, the oral channels open up again, and Paul no longer has a need to write letters.

This seems highly unlikely. A more likely explanation would seem to be that these "letters" were not actually occasional correspondence, but works of theological instruction composed *in the form* of occasional correspondence. This was not an uncommon practice in the ancient world; epistolary fiction was a popular genre, and authors were taught how to write documents that looked like realistic, genuine letters.