There's probably some benefits around blacklists too.
For example if a user uploaded questionable content to assets.example.com/uploads, such as pirated content then someone could submit that to search engines and other lists to get your domain blacklisted. It's quite possible these blacklists could be related to the apex domain, not necessarily the sub-domain. A separate domain guarantees your apex domain won't get penalized.
There's probably some benefits around blacklists too.
For example if a user uploaded questionable content to assets.example.com/uploads, such as pirated content then someone could submit that to search engines and other lists to get your domain blacklisted. It's quite possible these blacklists could be related to the apex domain, not necessarily the sub-domain. A separate domain guarantees your apex domain won't get penalized.