We have a shared channel too. It's great for times when you want to pull a client or a vendor into your slack for long term. You do want to be careful about getting pulled into slack (see this post and discussion from a week or so ago: https://news.ycombinator.com/item?id=20895853 ).
There's actually another use case where a vendor provides a bot and wants users of the bot to be easily able to file help requests. We wrote an app for that and outlined our thinking here: https://www.transposit.com/blog/2019.09.17-amazon-slack-shar...