Using #include to include sections of code

I think you're talking about OpenFOAM here. The problem that these code snippets solve is that of avoiding the duplication of boilerplate code that many applications in OpenFOAM need. Putting this code in a function won't solve the problem, because the variables declared inside a function are local to its scope. One could perhaps come up with a scheme of base classes that contain these variables as members. But that would just add another layer of indirection that doesn't really solve anything. You're still dependent on variable names (or, if you want to make it clean, getter-names).

Personally, I'm undecided on whether this practice is good or bad. It is how it is, and it is part of the OpenFOAM code culture (or local lingo, if you want). At first sight it's surprising, but one gets used to it pretty fast.

However, unless you are developing OpenFOAM applications/extensions yourself, I would strongly discourage this practice. OpenFOAM is somewhat unique in that it contains virtually hundreds of executables that all require some overlapping boilerplate code that would be hard to maintain otherwise. If you're not in that situation, then don't do it.