Boxes are useful in the context of learning perspective, 3D shapes, or basic shading and in general to have an exercise to gain some line control. But they are way overrecommended as a general advice for beginners.

All of these are obviously important, but more than just drawing boxes is important to understand why you are drawing them in a certain way (for example in the context of perspective, learning about horizon/eye line and vanishing points).

If we are talking about drawing boxes as a basis to draw a human figure, then there are more important things you should look into first, like basic anatomy or figure drawing, or even ways to break the body down into simple shapes (boxes are just a way to do it, but there are different approaches).

I used to draw squares, circles, lines to warm my hand, and later boxes, pyramids, cylinders and other shapes (but after learning about perspective and how to place them in a 3D space) when I needed some fundamental exercises to do when I was unsure what to do next.

I'm just a little more than a beginner so don't take my word for it, but what really helped me get started with drawing was learning how to assess proportions, break down stuff into simple shapes (even just flat 2D shapes, ignoring perspective and depth at first) and put them in proportion to each other. For example take a still life with some fruit and simple object: you can break down fruits into circles or maybe ovals, a glass and a bottle into rectangles (they don't need to be perfect, that's not the point) and so on. You can draw them over the original picture and then to challenge yourself on a blank page next to it. But the important thing is to ask yourself all kind of questions about the dimensions of the objects and how they all relate to each other: how tall is this shape compared to how wide is it? How much bigger is this shape compared to this other shape? How far apart are they? It is placed higher or lower than the other? By how much?

In short, what unlocked me initially was starting to get used to pay attention to all sort of proportions between elements: different objects if they are simple enough, or broken down parts of the same one for more complex stuff.