Currently, most copies of Excel are sold as part of Microsoft Office - a suite of products that includes a variety of other programs. (The exact programs that you get depend on which version of Office you buy.) Obviously, it helps if the programs can communicate well with each other. Microsoft is at the forefront of this trend. All the Office products have extremely similar user interfaces, and all support VBA.
Therefore, after you hone your VBA skills in Excel, you'll be able to put them to good use in other applications - you just need to learn the object model for the other applications.