It's not that the Seals, Delta, KSK, Grom, France, Etc etc use them it's because I know and most importantly I use them. I'm just kidding around a little. but seriously..
AR's come in 3 main classes for weight like most things with of course many specific requirements and variations of needs.
But to dumb it down.
Light
Medium
Heavy
HK's 416/417 fall into the last 2 groups mostly considering the more popular and main production variants out there. I would say there are different ways of arming your platoon or section, regiment whatever to get to the same result in many ways. But for professionals in many circumstances the HK line just meets the demands where others don't in durability, and usage. Of course this can depend on the variant, barrel length and type. Many bloggers/Interneters make comparisons between various companies and they are comparing guns built for almost identical purposes to the naked eye but within that orgs requirement it makes a difference.
Other thing to keep in mind some companies may make a fantastic rifle but how many can they produce? And then maintain that same quality? Most can't and many of these smaller companies are filling more specific niche which is great, but overall HK line wins.
The engineering on the 416 line really did take the AR to new levels and it's still the best out there for the most part.
If you have the cash I say get one for the history alone it is indeed worth it.