what is the logic /reason behind 50 or 60 Hz? Why not we go for Say 85 or  29 Hz i.e. for any random frequency?

Early mechanical machinery would only rotate slowly- in fact 25 Hz was initially used. Then 50 Hz/ 3000 rpm, and the latest machines 3600 rpm/ 60 Hz.
It also has an influence on the iron losses efficiency- the higher the frequency the less losses, like aircraft use 400 Hz to get the transformer weight down.

Higher frequency corresponds to higher iron losses.
Pulsating torques (from motors) can be avoided at higher operating frequencies.
Of course, lesser flux densities require less magnetic core material in transformers.

Chandra is right, higher frequency , more losses. My poor selection of English. However in aircraft the "cost" is in the flying that weight around. . So using a higher frequency allows less iron to be used, so less weight, (400 Hz transformer is 8 times lighter than a 50 Hz version) less fuel burn, but more electrical losses are acceptable in that cost /advantage balance ratio we all engineer to.