In June 2016, Instagram changed their API in a way that restricted access to third-party search engines and web viewers.
This was bad because back then, Instagram's built-in searching capabilities were severely limited. You could only search user names and single hash tags, but not more advanced things like descriptions, multiple hashtags, and date ranges. I don't know if they added it by now because I haven't used Instagram for several years.
Third-party tools like Hashtag Pirate had searching capabilities well beyond Instagram's built-in searching tool. From what I remember, Hashtag Pirate allowed filtering by type (photo or video) and date range, and allowed filtering by multiple hashtags. There was also a search engine which could search descriptions but I don't remember its name.
Third-party viewers like Websta.me, Enjoygram (later renamed to Pikore), InstaGravity, Instaliga (the few I remember) also were more lightweight and therefore worked more smoothly on older devices. Instagram's own website always relied on heavy JavaScript and consumed lots of memory.
Some also showed details about a post that were not shown by the Instagram website, like the exact date a post was uploaded. Instagram itself used to show "weeks ago" only, but they added exact dates in the late 2010s I think.
Third-party viewers also featured different layouts that may be preferrable to Instagram's own web interface. For example, Websta.me had a side-by-side view, meaning it used to show pictures on the left column and description+comments on the right column, and you could change the view ( I found in the archives).
The API was also used by bulk exporting tools like InstaPort.me, which also ceased to work.