In battleground states the win/lose margin will quite possibly come down to a few thousand votes, particularly in PA. The endorsement might not change anyone’s mind, but to her millions of fans it raises the elections’ importance for anyone who doesn’t “care about politics”. It turns a civic issue into a social group issue, and people care more about the latter. It’ll pull people reluctant to go to the polls to show up and vote.
US is a flawed-democracy, EU, Canada and Australia are currently the leading democrocies. That all could change but US is not the best in that area.
https://www.wikipedia.org/wiki/The_Economist_Democracy_Index