The difference between an autocracy and a democracy is that a democracy has the regulation mechanism of regular voting for political leadership. This regulation mechanism encourages political leaders to make decisions which are in the interest of the general public and not just in their own interest.
Democratic systems are built on the assumption that when people are unhappy with the political leadership, they vote for someone else. Not voting at all givessends the message "Everything works well, continue like you already do".
However, many people who don't go voting are often those who are the opposite of satisfied. They are often those who feel misrepresented by all major political candidates. However, whenBut if a democracy is supposed to improve, it is imperative that these people especially gothose people are voting who don't feel represented. The option is usually to vote one of the many chanceless minor candidates which usually run in elections. When the dissatisfied people spread their votes on the no-name candidates, they punish the major candidates. The major parties visually lose percent-points which will make them wonder what happened and if they might do better if they change their stance on certain issues.
So far the theory. But in practice, you have to look at who exactly tries to mobilize voters to vote no matter what. You will often see people behind such campaigns who expect that it will lead to more support for their favorite candidates. Others think that preventing people from going to vote can in fact be good for their political success, so they do not actively encourage non-voters to vote and do the exact opposite. For example, in Germany the current leading party CDU followed an election campaign strategy in the past years which was dubbed asymmetric demobilization by outside observers. It means they try to make the election as boring as possible by intentionally dodging all controversial topics. They know they have the largest amount of loyal core voters who will vote them on principle no matter what. So when they can persuade all the switch-voters to stay home at election day, they will benefit.