I can only explain this in a very amateurish way, but in Nextcloud there’s a lot going on beyond normal website requests or file uploads/downloads.
In addition to standard web traffic (HTML, JavaScript, WebDAV), there can be WebSocket connections to backends like Notify_Push, Collabora, Whiteboard etc, which means asynchronous traffic is happening all the time.
At this point, I must admit that I am not enough of an expert, and someone with deeper knowledge of NGINX, Nextcloud, and long-running connections would need to explain this in detail.
However, If I had to guess, such an expert would probably say that 86400s is overkill, and in the worst case could lead to resource problems or even security concerns?
But as I said, this is just my (not so) educated guess. 