I am new to fitness and will be doing load-bearing and high-impact exercises. It necessary to do warm-up exercises before starting load-bearing and high-impact exercises? If yes, then why it is necessary?
A warmup is a very good idea, but it doesn't have to consist of what you normally think of as a warmup. The warmup should consist of low-key approximations of what you'll actually be doing. So for weights that might be going through the movements without any weights, then with lighter weights, before going to your work set. For running that would be walking first. For baseball it would be playing catch. And so on.
The biggest reason for doing a warmup is you'll be able to notice any problems before they become serious. If you notice some unusual pain or problems doing a bodyweight squat, that's a good indication you shouldn't be squatting with a heavy load on your shoulders. That might just be stiffness, and if you do a bit of a longer warmup you work the kinks out, but it might also be an injury (from anything, could even be just sleeping badly) and you're best off just not doing that exercise at all that day.
From what I've read, warm ups are good for getting your body warmed up and your muscles ready for stress. Think of your muscles like rubber bands. When a rubber band is cold, it has a tough time stretching and is prone to snapping if you put too much pressure on it. Now, take that same rubber band and warm it up to a decent temperature and it'll stretch just fine.