One of Google's search algorithm ranking signals is social shares. Google expects a good site to receive a steady amount of social shares on a regular basis. Because Google knows that users who like a site will sometimes tend to share that site's content on their social media platforms.
If a website is receiving very low amounts of social shares, Google sees this as a negative ranking factor. It indicates to Google that the users it sends to your site don't like your pages very much, otherwise there would be a certain percentage of them that share your content.
Because most people on social media don't have dedicated channels to specific niches, Google understands that when a social media user shares your content, it doesn't have to be 100% related to their profile's general niche.
A social media share from your site is simply being counted as a vote by the sharer, as him saying "hey this site is good!". The more followers and activity that social media user has, the higher the ranking signal that Google receives. If a social media profile has 100k followers and a lot of activity, Google trusts that profile more than it trusts a new Twitter profile with 100 followers and no activity. And so the more trust it has towards the social media profile, the more trust it has in the social media share of your site in its ranking factor.
Getting shares on social media is a very good thing, regardless of almost anyone who shares your content.