SMS costs money and requires a phone. Some people cannot afford texting plans or might not have a cell phone at all (think a teenager). Even if you are capable of receiving SMS, they are destined for a single device: with these alternatives, you get the notification on all of your devices, including your computer (which has a real keyboard and is more convenient for longer conversations). The benefit of iMessage is then to solve these problems in a seamless fashion (assuming you buy Apple hardware, as that's how Apple makes all of their money: hardware): if someone with an iPhone sends a message to someone else with an iPhone, it just notices "oh, hey, this could have been an iMessage", and the result is cheaper and more convenient (as the message goes everywhere, not just to their phone); if the message cannot be sent via iMessage for any reason, it gracefully degrades to SMS.