If you do not set your frequency caps correctly you could end up showing your ads way more than necessary. If you show them too much, you might actually piss off the viewers, even cause them to report your ads or leave a negative review somewhere online.
We set our clients frequency to only show their ads 3 times per day and only once per site.
Once per site is crucial because sometimes a website has 3, 4, 5 advertising positions. If you don't cap how many times per site your ads show, then you might be showing your ads 5x on one site which really only qualifies as one impression.
Again, this just pisses the visitor off and is a waste of your money.