I like getting better. At anything. Who doesn't? Whether I'm getting faster at my job, or ranking up in a video game, or parallel parking like a boss, it feels good to improve.
This is especially true when the task is competitive in nature. When you are working toward a goal, testing your progress against others with the same goal is the only true measure of your progress. The thing is, it doesn't necessarily come down to your effort versus my effort - if it were that simple, skill could be quantified in hours, and we might as well go home as soon as we're up against someone who's been doing our task a little longer. It's also about knowing what to practice, where to get help, and how to respond to successes and failures along the way.
Dancing is no exception. And I suppose I'm mostly speaking in terms of competitive dancers, though I've met a few rather serious social dancers. Like anything else, you get back what you put in - however, "what you put in" refers to more than just time and energy. If you practice 10 hours a week, trying to duplicate what professionals are doing on YouTube, you will look like a dancer who spends 10 hours a week practicing what they saw on YouTube. If you take 3 hours of lessons with world-class coaches, practice on your own for 3 hours drilling basics, and only frame up with your parter for 3 hours to talk about connection and to dance rounds, you will look like a dancer who...well, you get the idea. 10 hours of unfocussed effort is not going to give you the same result as 9 hours of effective training.
However, where that focus goes is a tough question. I seem to come across two different schools of thought when it comes to practicing ballroom dancing, though I'm sure it translates to other skills.
First, I meet dancers that are focused on impressing. Flashier moves. Bigger shapes. More difficult footwork. Have a couple of amazing moments, and they're sure to want more. I used to belong to this group - gotta have a few go-to steps I could do well, and spam 'em. Focus on the strengths, and play to them. I have trouble with shapes? Then I'll do faster moves. My footwork is poor? I'll do the easier steps and more lines.
Thing is, there's only so much you can do like this before it catches up to you. Sorry, but the biggest frame in prechamp standard will probably get smoked if they're taking bronze-sized steps.
Second, there are the dancers that are attached to the cleanup. Learn every step in every syllabus. Learn all of the technique on this figure before moving on. "I have one section that doesn't feel right - let's work on it until it feels right." This route requires patience, but ends up being very rewarding in the long run. The hard stuff is easy once you've got the basic down solid.
And yet, where's the fun in that? And what if the problem you're stopping to fix isn't really the root cause?
I belonged to this group at one point as well. I spent weeks "fixing" my frame to make a single section of one routine feel a little smoother, only to find out much later that my footwork was wrong, making it nearly impossible to step without distorting myself (and my partner). Quick fix on the placement, and it was all good in the frame. And all that time was wasted - anything that I had improved upon was overshadowed by the time it took to get out of the bad-footwork habit I was solidifying.
Now I certainly can't suggests that either of these routes is the way to go - as you probably guessed, I prefer a mix of the two. In competition, you may only get a judge's attention for 3 seconds, and you need to make sure that you don't give them a reason not to mark you. You have to keep things clean. But just being clean isn't enough to make you stand out - you need to keep learning new concepts as well. In fact, sometimes the new stuff inadvertently fixes the problem spots - I had a big problem with timing in the Slow Foxtrot, but learning better swing helped fix that little mess.
Bottom line, if you show up to practice just to power-through your choreography 10 times, you might as well do it once and go jogging instead. Because unless you're actively working on something, then you're only doing you and your a partner a favor on the memory/endurance side. Instead, try fixing a bad habit you've internalized, or adding some technique you've learned but haven't internalized yet.
And remember, like most skills, identifying what to do next is easier with professional help. And knowing is half the battle.