Detaching is a useful feature for keeping some channel attributes (OP, topic, ...) for a channel while you don't need it at the moment. Also, this can avoid distractions from high-traffic channels when you still want to catch messages in other channels. To your client it will look like you parted the channel, but in fact you have not.
The autoattach module can re-attach you to a channel once there is activity.
Channels can be detached in two ways:
/detach #chan /quote detach #chan
A simple join will reattach you: