Why declare a variable or function static in C?

The keyword static has several uses; Outside of a function it simply limits the visibility of a function or variable to the compilation unit (.c file) the function or variable occurs in. That way the function or variable doesn't become global. This is a good thing, it promotes a kind of "need to know" principle (don't expose things that don't need to be exposed). Static variables of this type are zero initialized, but of course global variables are also zero initialized, so the static keyword is not responsible for zero initialization per se.

Variables can also be declared static inside a function. This feature means the variable is not automatic, i.e. allocated and freed on the stack with each invocation of the function. Instead the variable is allocated in the static data area, it is initialized to zero and persists for the life of the program. If the function modifies it during one invocation, the new modified value will be available at the next invocation. This sounds like a good thing, but there are good reasons "auto" is the default, and "static" variables within functions should be used sparingly. Briefly, auto variables are more memory efficient, and are essential if you want your function to be thread safe.

There are a few reasons to use static in C.

When used with functions, yes the intention is for creating abstraction. The original term for the scope of a C source code file was "translation unit." The static functions may only be reached from within the same translation unit. These static functions are similar to private methods in C++, liberally interpreted (in that analogy, a translation unit defines a class).

Static data at a global level is also not accessible from outside the translation unit, and this is also used for creating an abstraction. Additionally, all static data is initialized to zero, so static may be used to control initialization.

Static at the local ("automatic") variable level is used to abstract the implementation of the function which maintains state across calls, but avoids using a variable at translation unit scope. Again, the variables are initialized to zero due to static qualification.