Maybe a stupid question, but why is the case grounded at all (I mean as in "put on a common zero potential", not as in "connected to earth")? I imagine it is co

Maybe a stupid question, but why is the case grounded at all (I mean as in "put on a common zero potential", not as in "connected to earth")? I imagine it is convenient to use the case as common ground, but it seems wierd. Is is unavoidable production-wise?