Don't do machines, they are really not that good for most uses. The fringe usability I can think of - competition bodybuilders, training around injury of professional athletes. That's probably it. Anything else ain't worth it.

People often do max 1 specific muscle with them, ie biceps or chest to impress and fight low self-esteem, but that's generally not a good way either.

As mentioned, muscles work in complex groups, and having 1 part way stronger than the rest will lead to injuries in real world usage. And injuries mean you will lose it all, and possibly worse.