To offload the storage requirements of the instance you’re posting in. If everyone starts uploading images directly in the comment, the server HDD/SDD will be overloaded. Consider using a third party service and posting the link from there instead!

Hosting the images directly on the lemmy/kbin instance creates imo unnecessary traffic to the servers. I don’t know if admins can disable selfhosting images, but considering the low-powered machines almost every instance is currently hosted on, I would assume it would be in everyones self-interest to outsource image hosting (at least for now).