Tinder's more of a matching service (you literally swipe left if you're not interested, right if you're interested, and the app tells you if you both swipe right, at which point you can message one another), while grindr uses some kind of geolocation, not to mention that, from what my roommate showed me, it seems to have some sort of social media type functions (although I'm not too sure about that). Plus, you have to consider the economics of it, for a lack of a better term--dudes, both straight and gay, are typically hornier than women, and some gay/bi dudes are probably in the closet and don't have any other way to hook up or meet other dudes.