It can help, for sure. I think the main reason it typically isn't enabled in general is that it's pretty common nowadays for people to switch IPs regularly. It'd be annoying to have to re-login to Slack multiple times per day as you move around with your phone or take your laptop to different places.

(Especially if it's a user who isn't in a corporate environment, where your public IP will probably differ when you connect to different wireless networks. On a corporate network you're more likely to retain the same public IP no matter which room or building you're in.)

If they don't, they should provide an option for corporate administrators to enable IP locking for session cookies.