Cliches like "You get what you pay for" and
"If it ain't broke don't fix it" immediately come to mind.
You seem to have answered your own question in most aspects.
Intelligent use of the answers you get from some software may help you to decide if the highlighted matter is a problem or not.
Diagnosis is one thing and there are many good(free) programs which will tell you things about your computer which otherwise are difficult to find.
Software to repair of systems - particularly if damaged through some hardware flaw, is not only possible but is used by many repair shops to save them time when rehabilitating a thorougly messed up PC.
Such software comes at a price to the repair outfit and while it allows rapid repair of really difficult situations it is only worth buying if the user expects to be doing a lot with it for a suitable fee.
A bit like the sort of computerised diagnosis kit that franchised garages have to buy to be able to find a way through the engine management and other problems which hit today's cars sooner or later