Yes, of course, they benefit hugely from the ability to select students. They're also less answerable to parents or the public than public schools, in many way, which can free them up to ignore stupid demands from parents (they select families, as much as they select students)—or free them up to maintain stupid policies or approaches that could never fly in public schools, which is sometimes exactly what their customers want (as in the case of e.g. fundamentalist religious schools). They're more answerable to the set of expectations they've created for their families, than to the particular whims or desires of any given family, or any constituency present in the broader public.

... which doesn't necessarily mean preferring a better school that's only better due to selection bias (and knock-on effects from being able to remove very-disruptive or low performing students) is a bad way to go, at the individual level, if one has that option. The observation's more relevant to policy-making, than to the relative merits of those options to any given family.