Dear

 I not have answer, but it is evolution evolved behaviour.

It might be trigger some type stimulation and hormones secretion in male.

You might get answer though, usually people kept common hens & cocks in small cage.

When it open all the hens run fast due to that action cocks motivate run behind the hens and met. But when it in cage male not take interest in hens.

Best

Raju vyas

Thanks Raju Bhai

Probably a test of male competency. Female snakes will often move during coitus. 

Thanks Dr Schuett. I will be highly obliged if you can send me some references on the mater you cited.

Sanjay

moving in copula is not particular for reptiles, it is more often in amphibian birds and mammals......enjoying sex calmly is only human....

thanks prof Khan

By vulnerabiliad to predation

Which lizards have you witnessed moving while mating (I presume you mean travelling, rather than just wiggling)? Is it possible that they were influenced by your presence? Varanids don't travel while mating and neither have the wild agamids I've seen here (Australia).

lizards might be doing moving of tail during mating so as  to maintain the body temperature with the surrounding, Since lot of energy is used up during mating. 

Thanks to all for your valuable answers. Can anybody send me some literature about breeding behavior of reptiles.

will explore