The biggest benefit of .internal IMO is that it is free to use. Free domains used to be a thing, but after the fall of Freenom you're stuck with free subdomains.
If `.internal` is for private-use only, they must be resolved by some sort of private or internal DNS. In that case, all domains are free for private-use anyway.
Unfortunately, that's not true in general. Google proved this with their handling of the .dev TLD. Security settings like the HSTS preload list can impact your internal network if you "squat" on a domain you don't own. Google added all of .dev to the HSTS preload list and now, if you use any domain under that, you browser will force you to use HTTPS.