When I look at car ads with pictures the seller sometimes cover the number plate. What is the purpose with that?

The owner wants to stop anyone from "cloning" the car i.e. pick a similar colour and model put false plates on it to avoid paying congestion charge, speeding/parking tickets or worse.

So when you are out driving on the streets you cover your plate so no one can clon them? I mean that's as easy as seeing a car in a car ad.
I don't see the point.
Is it because the plate has no connection with the chassis number of the car?

You wouldn't want to clone a neigbours car! Much easier, quicker/cheaper/greener to go out of area onto Auto trader or whatever.

If you want a clean HPI provenance as well (for resale) pick one from dealer forecourt

simples