but, we can be individually set to "Don't block", "Alert", "Block and notify" or "Block silently"
why by default is set to "Don't block"?
because, it's for advanced users. which want modify many settings with details.
the function of that is protecting us from suspicious websites when we browsing on the net.
for me, I set to default and No problem.
Ad/tracking hosts - Hosts used for advertising and tracking purposes.
I think its on Don't block because a lot of websites have various ad services embedded in web pages and the process isn't really malicious so having it on Alert or Block and notify/Block silently would cause more problems than benefits.
My advice would be to leave it on Don't block as I'm sure a lot of site use ads or tracking services.