and to get more adepts. simply having more cosmetics around should encourage other people purchasing cosmetics too. also, individuals who would ignore cosmetics altogether might indeed get interested if they are free, and are more likely to eventually make a purchase (and here's where those odd pricing schemes come in too).
in these models usually a very tiny portion of the population buys a lot and constitutes the bulk of the revenue. you can't squeeze much more out of that, what they spend is already ridiculous and there's just so many of them, so the sensible thing is to try and entice new candidates. you do that by making paintjobs be a thing for them too, by giving them a few for free. an exaggerated example would be handing out drugs in a school.
it's also a 'positive' feedback loop: the more you invest in the game (hardware, cosmetics, grind, community content, whatever) the more perceived value it will have for you, meaning the more you will desire to play which in turn motivates you to invest even more. those free paintjobs can suddenly have value for players that wouldn't have bothered before, and could contribute to start and keep that ball rolling.
