Farmer's Walk variation?

[sax]

Hey guys, at the gym today I saw a man doing farmer walks, but he had a 50lb dumbbell in one hand down by his side and a 25lb dumbbell in the other hand with his arm raised above his head. I asked him what the deal was and he told me it helps build better core strength. Can someone explain to me if this is legit? and if it's actually a good alternative to traditional farmer's walks?

Also, when is it best to incorporate farmer's walks? on a core day? Or when I do an upper body day? How influential are they to muscle development in general?

Thanks guys! Cheers!