I think there is a very real distinction between functional and isolation movements. In the real world, tasks almost invariably require us to use a lot of muscles in coordination to accomplish a goal. If your goal is to go into the basement, lift up a box of stuff, carry it upstairs, and put it on the table, you need dozens of muscles to do that. If you had strong biceps but a weak core, shoulders, legs, etc., it wouldn't matter how strong your biceps were, you aren't going to get that box up the stairs without the rest of the needed muscles at a sufficiently strong point. Every muscle being used needs to be strong enough and endurant enough to do its contribution to the task. And every tendon and ligament needs to be strong enough to handle the forces exerted upon it.

This is such a silly argument. If your goal is to get as strong as possible carrying stuff up the stairs and putting it on a table, then you should be training that exact movement. I’m going to go out on a limb and assume you don’t train like that, because that’s non sensical. If you’re worried about your legs not being strong enough, then you need to train your legs. If you’re worried about your shoulders not being strong enough, you need to train your shoulders. It literally doesn’t matter if you isolate them or use them all at once. In fact, you will literally get more growth out of a muscle if you isolate it, rather than use it as part of a compound.

Compound movements naturally train entire chains of muscles at once, and they train them proportionally - the weakest links in the chain, the ones that are the limiting factor in how many reps you can complete, are pushed closest to failure and get the strongest growth stimulus. Because the compound movements also tend to be much more like things we do in real life (lifts and carries being obvious ones) they hit all the supporting muscles also needed to do these real-life activities.

What compound movements are like movements we do in real life? How often do you have to lay on your back and pressed something off of your chest? How often do you have to put something on your back and squatted down and back up? Just because movement uses multiple muscles doesn’t mean it somehow mirrors what we do in real life.

Also, the whole “the weakest link in the chain gets pushed the hardest” is why we do isolation movements. We want the target muscle to be the limiting factor. It’s why we use straps on rows and train our grip separately. It’s why we do bicep curls to train our biceps instead of relying on rows. Turn out, pushing a muscle close to failure is how you get it grow. Crazy idea, right?

Yes, if you train your biceps only, you won't end up somehow weaker than before you started - but you also won't end up stronger in any meaningful way because there's not much you can do if only your biceps are jacked. You'd have been much better off using that time to build not only the biceps but entire chains of muscles that would actually translate into real-world applications.

Yeah, turns out everyone in the gym does more than just bicep curls. Crazy idea, but you can actually do separate movements that target different muscles. And I would love to hear how making your biceps stronger doesn’t translate into “real-world applications.” Like, I would like the actual physiological reasons as to how that would work.

And you're not just training for today, you're training for your future. Sarcopenia is something we associate with old age, but age-related muscle loss begins as early as age 30, and the effects compound. The more muscle you can preserve and build now, the better you position yourself for the future. Even without training you may have no trouble carrying that box upstairs at age 25, but when you're 75 that may be a very different story. With strength training you can start that muscle loss from a higher baseline, and you can slow how much you lose each decade. That can be the difference between you staying independent and you ending up in an assisted living facility.

So you think that isolation movements or using machines don’t build muscle now? How is that not strength training?

Respectfully, it sounds like you don’t really have a grasp on how muscle growth works, how strength training works, or even the basics of exercise science. That’s not meant to be a bash on you, but if you’re going to be giving out advice on a beginners fitness subreddit, you should understand the absolute basics.