can any one tell me why places like roswell area 51 amongst numerous others are built on dry lake beds, what is the relevance of this
thanks

For the very obvious reason of the location being in the middle of nowhere with the least possibility of being seen.

the lake beds are flat, and make good runways for the experimental aircraft tested there... and its location is very isolated...

A large air base exists on the southwest corner of the lake (37°14′N 115°49′W) with two concrete runways, one of which extends onto the lake bed, and four unprepared runways on the lake bed itself.

Area 51