Is combinatorial novelty without insight useful? Who cares if we're the first to use tool T on problem P?
Is this valuable to science? If so, why?


Because it leads us to understand if tool T works on problem B. How big the "insight" that we gain from this is depends a lot on how different T is to other tools that have already been used on B, or, conversely, how different B is from other problems that T has been applied to.

The range here goes from "it is mind-blowing that T could work on B" all the way to "meh, everybody knew that T would work because we use it all the time for B' anyway" - although I will grant that most works following this schema in practice end up more on the rather incremental side of things.