Imgur now blocks several VPNs and have issues loading embedded previews in several fediverse platforms. So instead of using imgur, you could use one of the following alternatives for uploading your images.
https://postimages.org/
https://imgbox.com/
https://imgbb.com/
https://www.imagebam.com/
Can somebody explain on the purpose of these sites?
The whole time when I was using reddit I would just upload from my gallery to the app, never had to use an image uploader website, it sounds like a pain to use.
Using them do add one or two extra steps before posting. Images can hog up server resources and using these third-party sites reduces the burden for the server of your instance which is run by volunteers/hobbyists with money often coming from their own pockets. Its just a nice thing to voluntarily do.