In my opinion, for example for the back, there is no anatomic or logical or topological reason for the current edges disorder, the human torso is a kind of cylinder, so the edge loops should follow the shape in a much more logical way, like that (nota bene: it's not a detailed drawing of what the topology is supposed to be, just a rough description of a better edge loop flow):
The main advantage is that it will be much easier to rework or to give additional edge loops if you need to. Also, as Susu says, it's important to have good topology for animation because the mesh will need to deform properly.
The second thing is that I doubt all your edge loops are useful, for example here:
An easy way to know is that if you ever dissolve an edge loop and it has absolutely no effect on the object appearance, it means that this edge loop was unnecessary. For the knees and all the joints though, you'll need a bit more density because it will need to bend properly during animation.
As for the density in general, I don't think that it's a problem to have various densities as long as it is justified, the face for example will need a much more dense topology than the thigh.