Not to mention pre-order bonuses. Why the hell would anyone per-order a digital game, where there's no chance it'll sell out and they won't be able to get a copy? Dumb-ass pre-order bonuses, I guess! People buy them! What the hell, gamers?
I did that for Borderlands 2 because I liked the first part so much I was willing to gamble on the second one being good. I was not disappointed. Of course I'm aware it's a gamble, which is why I tend not to do it, but sometimes it just might be worth it. Oh, and because the first game's German release was censored I imported from the UK so pre-ordering cut down on the waiting time.
And, of course, streaming and "let's plays." Why are people sitting around watching OTHER PEOPLE play games that they themselves could be playing? But they do!
As has been pointed out, not owning the console is one thing. I'm not going to buy a 3DS for a single game. Or perhaps you no longer have the game and want to take a look at it for nostalgia's sake. Some games may also be interesting from a story perspective but uninteresting gameplay-wise - I might end up watching an LP for Mass Effect 2 and 3 but I'm certainly not going to install the second part again, much less buy the third one. Some Let's Plays are value-added like IlliterateChild's Glitchy Walkthroughs where bugs in the game are exploited for comedic effect.
Sure, playing the game is usually better. But there are games you can't or won't play but are still interested in experiencing to some degree or someone did something with their playthrough that you can't easily replicate. That's where the value in Let's Plays lies.