Because it increases sales.

logical ads

Advertising is the only reason to do so.

Yes it does. The reason for this is because in advertising you need skills in the visual field. Visual communication has a lot to do with design, and advertising has a lot to do with design. So since they are a like in many ways the two together would be perfect or advertising.

The reason why every answer-page is overloaded with advertising to the point of not being able to get answers is that the host of the website allows.