Essentially by default, Live can take any material and automatically ensure that it matches the tempo (and key) that you're working in. Clip launching is a very powerful way to play around with musical ideas, even if it is not necessarily the best way to produce finished pieces of music (though it can be). Reaper doesn't have this workflow (at least, not builtin). It is this "I can goof around with almost any material and it almost always sound good, or at least interesting" element that I believe made Live so popular.