Host discovery

What kind of target makes sense to use host discovery?
I did it to my website while ago, hosted in those hosting from TV commercials.
There where a few Up, but none of them was mine.
So, make sense to do Host discovery on small business?
Isn’t make more sense to do it once we’re inside the target to find other potential targets in the “”“intranet”“”?
I guess that a big company maybe take a range of IPs to host different services or portals, so in that case makes sense.
But I also guess that it couldn’t be allowed if it’s break the rules of engagement. So, I guess it’s tricky.