I think the idea is to observe that these kinds of systems share many common components and internal functionality. This is obvious to anyone who has built more

I think the idea is to observe that these kinds of systems share many common components and internal functionality. This is obvious to anyone who has built more than one of these things. It therefore follows that it might be interesting to build a general purpose toolkit comprising the common parts, allowing specialization as a layer above. This could for example make it easier to create new categories of query processing system because less foundational work is needed.