This little bit of rubber is designed to ensure you can get a 28.5mm caravan waste hose connected to its fitting with a water tight seal. It sits inside the end of the hose which then creates a tight, push-on seal against the associated plumbing fitting. Using one of these limits the need for a jubilee or hose clip as the fit is very snug. You can use a jubilee clip if you like but you’ll end up crushing the hose a bit which just creates headaches. If you need to unblock the sink for any reason these are handy fittings to have fitted as you can , with some gentle persuasion, yank them off rather than faff around with a screwdriver or socket set under the sink. Basically they save you a few plumbing nightmwares.