One of the biggest differences between the two types of practice is that one is focused, specific, efficient and necessary for development and the other is, well... none of those things I just mentioned.
Here's the thing... If you were to practice a song by singing the song over and over again, or if you were to practice a monologue by simply reciting it until your jaw hurts, are you really making any changes? Are you ACTUALLY developing? Sure, you might be more familiar with the material now; you will certainly know it by heart at this point, but if we're trying to change and develop our skill level and if we're trying to actively adopt a new habit, which is better than the old habit, then this is really not an effective way of working.
The human body is an amazing thing. We have this wonderful function known as 'Muscle memory'. This function gives us the capability to train ourselves to do something a certain way without having to consciously think about it. For example, breathing... we don't walk around as humans thinking about how long each inhalation should be and when we should exhale and whether it will come out through our mouths or noses; this is something that our bodies have learned to do over many years of adaption and evolution. We can all agree it's a blessing to have this function; however, it is also a curse... let me explain...
If we have been doing something in a particular way for many years, like singing for example, it means that our bodies will have found the most 'comfortable' and what your body believes to be the most 'efficient' way of singing. However, if you have never had professional singing lessons or coaching, then there is no way your body is even aware of what 'efficient' singing is... good singing is always about proper breathing; if you can master breathing, you're more than 50% on your way to being a much more efficient singer. So, here's the hard part, when you suddenly tell your brain, "Hey brain! You know that singing thing you've been doing for years and years? And you know how you think you're supposed to breathe and how you're supposed to use your jaw and your tongue and what not? Well... I hate to tell you, but you're wrong." Do you think your brain is going to just say, "Ah, darn! Well no worries! I look forward to simply switching the way I've been doing things for years and years overnight, because some bloke sat at home on the other end of a computer is saying I should"? The answer is a very big "NO!"
Believe me, your brain is going to put up a big fight! It's a survival instinct! You could say that our bodies are a bit stubborn and a bit lazy... Our bodies don't like to change because in order to change we have to work very hard and sometimes, working hard makes us tired, stresses us out, puts our bodies and minds under pressure; think about training in the gym... unfortunately, you can't just lift weights for five minutes and hope to look like Dwayne Johnson in one weeks time. You have to challenge your body to change. You have to offer it stimulation and a reason to grow and develop. Learning and developing a new habit and/or skill is exactly the same thing! This leads me to my original point about practicing a song by singing it over and over again; this is not good and effective practice, because you're not actually changing anything. You're simply singing the same song in the same way over and over again. In order to change the way you sing that song (maybe you want to hit a particular high note within the song more clearly, or there's a particular phrase that you struggle to breathe through), you need to actively work on those areas. You need to break it down and really focus. Maybe you'll need to apply exercises to a particular part of that song- for example, if you want a cleaner high note, you may need to take time to mobilise your larynx and improve vocal set up before you sing that particular note. You may need to isolate that one part of the song and instead of singing the whole song over and over again, you may just want to spend twenty minutes on one section that actually needs improvement and development. You can see why this might become stressful and mundane; focusing on one specific note for twenty minutes instead of having the joy of just belting out your favourite rock song over and over again... honing in on what your breathing is doing or what your jaw is doing before you hit a particular note instead of being able to just freely sing with careless abandon. But, this is the only way to effectively practice and make a change to your voice!
Here's the good news.... you don't need to spend hours upon hours at a time in order to make practice effective; in fact, I would actively encourage you from doing this... If you spend three hours agonising over one note or over one phrase or over one vocal exercise, not only are you going to knacker your voice out and therefore hinder yourself from being able to use it, but you're also going to set up poor associations with that particular note or phrase or entire song! Every time you sing that song, your brain will recall those stressful and agonising practice sessions and you won't be able to sing it with full confidence and comfort- just another example of the curse of muscle memory.
Instead, all you need to do is spend fifteen minutes at a time on a particular exercise or on a particular habit that you are trying to change and develop. Take breaks, drink water, work in a non-stressful environment, don't practice until you feel like you have razor blades in your throat or until you're crying from all the pressure you're putting yourself under. Effective practice is all about achieving your goals efficiently; it's a marathon not a sprint. Take your time, adjust your focus, control your energy and you will actually see the results!