Many applications, from LibreOffice, Logseq to video players could all benefit from having social functions to compete with centralised solutions like Notion, YouTube etc. Expecting each of these to implement servers for comments and annotations is not realistic. However with a self-hosted annotation server, these apps would only need to implement one integration and users would only need to be connected to a single server.
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The code for both the client and server are open source (https://github.com/hypothesis/h) so this is possible. The server is designed to support the needs of large scale deployments, so this does come with some complexity compared to a system you would design for smaller scale usage.
The text on https://web.hypothes.is/ mostly targets schools and universities, because Hypothesis pays for itself by selling integrations with online learning platforms (Canvas, D2L, Blackboard etc.) and associated support.