I have never used Instagram, what is its purpose?
To look at pretty pictures.
You can follow (subscribe to) people/accounts that post content that interests you so that you see it when you open the app or visit the Instagram site. You can also follow hashtags- Example: somebody might post nothing but pictures of their food or kids or something else equally un-interesting to me, so I may never even see their account, or would not follow them if I had seen it. But when/if they post something tagged #stovebolt or #classictruck or some other hashtag I have set up the app to follow for me, then I see their posts with the hashtag that interests me, without having to follow that person's account and see all the rest of their stuff.
I follow a lot of hot rod, classic truck, vintage hot rod photo, bomb lowrider, Fox-body Mustang, and themed accounts, and don't have a whole lot of people I know in real life because I use Facebook for that.