Just as some console gamers own both consoles (and probably a PC too) some PC owners will want to buy one of these, Valve are certainly making it as attractive as possible for them to do so. By convincing as many Windows users as possible to run SteamOS/Linux (dual booting be it for older games or just for general use) and/or buying a Steam Machine they are presumably trying for as close as they can get to a single market of about PC + one console in size which for any games company but especially for the smaller ones would be very tempting.
Plenty of what-ifs and strong (possibly misplaced) optimism but at any rate, the only guarantee is that if they don't try then they will fail.
It is nice to see some enthusiasm and genuine innovation in amongst the usual world of marketing driven sequels and sure bets. What a shame that there are so many nay-sayers pissing all over it given that failure is as likely to be because of the nay-sayers swaying enough of everyone else as because of any inherent flaw in Valve's plans.