facebook pixel, when installed, starts collecting all the fb ID's of people that visit your site. Period

So WHEN you decide to advertise better than a boost (which sucks and wastes your money) then you can use your target audience as the people that visited your site but never reached out.

So if you dont have it on your site collecting potential clients you are heating a house with the windows open by letting good potential clients' fb id's not getting collected for when you when at scratchers or your grandma's will kicks in.

Its super easy to do.