hex stripping out of the head

16:01 Jun 20, 2018 

19M8406 socket head cap screw M6 x 1 x 20 mm
and
21M7632 button head cap screw

(takes 15 sec googling)

you will see that BOTH need an Allen key, but the hollow hex profile is much deeper on a "19M8406 socket head cap screw M6 x 1 x 20 mm" so it won't be as quickly damaged, as the effort will be spread on a larger surface when tightening it.

BTW, it's only the hex hollow profile of the head that gets damaged, not the rest of the screw.

11:06 Jun 20, 2018 

This reads to me as though the "hex" is a hex spanner that was used on the original " button head cap screw" and that stripped it, possibly because the spanner was too big. Replacing it with a "socket head cap screw" means that the offending hex spanner can't be used again and they have to use an Allen key or similar.