I think it's mostly a brand awareness/marketing type of trade off. You get your pi hosted for free, and then you are impressed with their uptime, service, whatever, and you are more likely to recommend your friends use this company's services, or get a paid vps or something from them yourself next time you need one. I took advantage of edis's offer of free pi colocation in the past, and since then I've also purchased a vps from them, and had several friends do that as well.