[crazygringo]

It's a good business practice because it's good branding.

Companies intentionally try to associate their brand with other things seen as positive -- pride month, the Olympics, the environment, promoting diversity, etc.

Google does this with their home-page doodles. Encouraging people to vote is another feel-good brand association.

There's nothing nefarious here like Google promoting a specific political agenda, that's overthinking it. It simply improves their brand image which makes people more likely to trust and use and promote their products. It's the profit motive at work... which, to answer your question, is why a for-profit advertising company does it.