I found that most of the websites disallow
mediapartners-google in robots.txt.
Why do I need to do block
mediapartners-google? Can blocking it help withp better SEO or does it not matter for SEO?
It doesn't matter at all in SEO, because it's AdSense crawler
If you've AdSense Ads in your site, then you shouldn't block
mediapartners-google user agent in robots.txt. But if you don't serve Adsense ads in your site, then you can surely block it just like other webmaster do it. If you see such a user-agent in your website log and if you think it consume more bandwidth then you're free to block it. But they have already configured their bot very well and crawl required and important pages only.