For Nintendo, it was the only way to upload your screenshots to the internet if you don't have a PC or phone with an SD slot. The other consoles have other integration you could use for images (and they all have YouTube for video) so this kinda sucks for Switch owners that have literally no other way of getting their screenshots online.
The only place where I tried to use it was on Xbox back when Xbox One first came out, and I didn't like the way it worked back then, so I didn't really use it much. It didn't send the actual media to Twitter, it posted a link to the file, and Xbox screenshots got deleted after 30 days. If I wanted to properly post it so that the media was actually hosted on Twitter, I had to save the full res media anyway.
(In fact actually saving full resolution Xbox screenshots used to be needlessly difficult. Only much later they added a way to save screenshots to OneDrive, which occasionally worked, and only very recently they decided they don't bother with the Xbox screenshot hosting at all and auto-upload everything to OneDrive.)