You already described what it is. The use is that it's a low-level language that works across all browsers, is quite fast in most and very fast in some. What you make of this is as open-ended as what you do with any other programming language.
- In the same vein, porting existing JIT compilers to run in the browser (cf. porting interpreters, with effectively nil overhead over JS). This is probably only feasible when JIT compilers are automatically generated, as with PyPy.