What's the difference between a Slack channel's purpose and topic?

From the Slack documentation:

Channel topic

The channel topic appears in the channel header, and anyone in the channel can modify it. It's a quick way to let people know what members in the channel are working on or important dates like project deadlines.

Channel purpose

A channel's purpose is a short description of the channel's intended use. On desktop, it appears when members browse channels and in the channel details pane.

So the purpose is supposed to be the overall long-term purpose of the channel, while the topic is meant to be used to communicate more current information.

In addition there are also sticky posts, acting like some kind of second-class topic as well.