When I buy dvds, do the advertisement budget added to my films or is the budget only for the studios? Is it worth for producers to buy them?

DVDs help the ad budget on your films and can be used on every release you do (so they can be used on films you do at all studios, not just a specific one). So, in essence, the more DVDs you buy, the more they help.

Also, once you buy a DVD, you cannot buy the same DVD (So you can't own 2 copies of Inside Man, for example).

DVDs do not count as an item but have there own section in your house which are unlimited.

It is worth to buy them and other awards that increase ad budget, as the more advertising budget you have (currently the cap is 20 mill with just cash), the higher amount of money your film is to earn.

Small correction:

Awards do not add to ad budget. They only replace the cash you would normally pay.

Studio filmographies, producer filmographies and item based ad budget actually allows your ad budget to exceed 20 million.

On full release films it's 20 million from cash or awards and 20 million from filmographies and items. Thus 40 million is the max.

For Events it is 15/10 for a maximum of 25 million

For Arthouse it is 10/10 for a maximum of 20 million

Ad budget has a big impact on the box office formula, especially in the crucial early weeks. And many established players have the option of maxing their ad budgets on every film. So catching up matters.