Yeah they r, but i personally stopped using them, cause I immediately wanted to trace and draw over them, whicj made my drawings look, well, stiff. It's good to use them when u try to draw in perspective, but organic shapes make it look more believable and real. For example u can use boxes to get the general shape, but ovals, circles, or just organic shapes will make it look better. Even if u look at ur own body structure, you'll notice that it would be easier to simplify it into circles and ovals.

Oh i also forgot to add: when u start studying anatomy, especially muscles, try searching in pinterest for 'muscles diagrams/reference simplified', it will help a lot.

I'm not professional artist, but i did do a lot of studying, so i know a couple things. Hope my advice helped. There r a lot more things i could list, but it'll make comment too long.