Many reasons. The main ones me, it's because I can chose a client that fits my needs among the many mature clients. And it's very configurable, as well as scriptable (though I haven't needed scripts for a while).

Most clients store the history locally in plain text files, which makes them easy and fast to search in with grep.

> is it encrypted?

The short answer is no. Many clients support E2EE using an OTR (https://otr.cypherpunks.ca/) plugin but it's not very user-friendly and your recipient is unlikely to have such a plugin already installed