I think it's more to get as much context as possible. One of our older products just sets window.onerror [0] to a small function that sends the information back to the server, which then forwards it to Sentry. The information in Sentry is nowhere near as detailed as if it came from Sentry's library, but it's been enough for us and doesn't seem to miss anything.

[0] https://developer.mozilla.org/en-US/docs/Web/API/GlobalEvent...