What they care about is if you have given them a solution that solves their problem(s). They don't care how you got there. They don't care what technology you used. Just as long as you solved the problem(s) they were having, and that you did it in the time/budget allotment for the job.
That said, as the solution provider you should have found out both their current use cases and potential future use cases, in order to select the tools you'll need for the immediate solution, and ideally for any updates or improvements that may be needed later on.
Because they will care, for instance, if the code you wrote was in a language that won't allow them to scale up quickly and easily (assuming you wrote the rest of the system to allow for this of course), or has some other kind of issue that prevents the client from moving forward with a change.
The last thing a client wants to hear is "complete rewrite using different language/framework/system" just to implement an extension or upgrade path (the best you can usually get by with is a refactor of the existing code base - but you better have a damn good reason and plan for that refactor).