Oh you don't "need" it I just see it as a fun photo sharing site. There are 10-12 filters you can use to alter / enhance your pics (with just one click) I share some pics there that I don't on FB.... just a smaller audience.
If you add a hashtag to your pic, then you/anyone can click on that & see all the pics that others have hashtagged the same. For example, I post a selfie & say "check out my new #sunglasses " Clicking on that would show you a bunch of other people's sunglasses - LOL
Sometimes couples will create their own hashtag for their wedding, then anyone with Instagram account can add that hashtag to pics & they'll automatically go to that one link. Britt & I did that for our last vacation together.