Let’s say you’re on a 100mbps connection and you would like for the browser to be able to use a maximum of 50mbps. I think that a built-in feature like this is really helpful and not that hard to implement by using a simple slider or a dropdown option for percentage of the total bandwidth.
By having the option to limit the bandwidth that the browser uses you can prioritize other processes like steam/video streaming service/apps etc. outside of it.
I’ve also seen browsers implement a cpu/ram limiter and that would be an added bonus, but can decrease stability for an inexperienced user.