LLVM being a little faster is nice, but that isn't the biggest reason for its popularity.
1) IDE developers (well one) that don't wish to be GPL compatible needed a compiler that communicates semantic information to the IDE that they could integrate without infecting themselves with a GPL license. clang/llvm exists for this.
2) API and JIT. If you wish to generate code programatically this can be a win. I've written functions to temporary files, forked a gcc, and dynamically loaded the objects, but it isn't the fast way and it isn't the clean way.
3) LLVM is modular and agile. Extend or twist it fairly easily. Try to twist gcc and it may take your hand off.