It depends on what you mean by worth it? It is a good program.

When you say "App," I am thinking for use on a tablet or phone. Word 2013 can be used on such devices, but I haven't tried it myself. Other than the mobile apps, I have seen no good reason to use Word 2013 over earlier versions.

If for use on a computer and your only real use of it is for proofing, I agree with fumei that Word 2003 will probably meet your needs as well.

Generally, Word is purchased as a part of an Office suite of programs. It is comparatively expensive when purchased as a stand-alone product.