You just cannot make generalizations regarding "breeds", you must speak of specific strains with which you are familiar, have seen, have raised and perhaps have even shown. Some strains of White Rocks lay very well for a heavy, pretty decent meat bird. Other strains are tiny and not bred to the Standard and are merely Leghorns blooded commerical layers posing as White Rocks. White can cover just about anything. They will lay you up a storm of eggs. It is all what the strain has been bred to do.
All that said, there are indeed lines and strains that utility has NOT been a priority, showing has been the focus for many generations. It is a buyer beware world, caveat emptor. Know the strain of birds you're buying and know what your plans are for them.
Strains are within varieties, not within breeds (varieties are within breeds). Lines are within strains. Granted, there are not that many true lines or strains in poultry now, although people still refer to such. Most people starting out cross strains, so they don't have a particular strain at that point, although most like to refer to a particular strain they started with for marketing purposes.Breeds are not species, of course, so since breeds are man-made, strains within breeds are even more so. Humans doing art and science projects. That's pretty much what breeding is.