you can use them to get a more natrual feel to the way your character is built this also aplyies to the skeleton, use it as a guide to make it look more normal and not like they dont have ribs this can be tranferd to any style and not always super realistick, just keep in mind how they are built

you can use these to get a feel for how your character moves and it´s usefull for lining everything up, it will make your characters look less stiff

a lot of times, i see sunken or enlarged heads when people dont consider how big hair actually is, some hair is actually really big and some is really thin but when doing certain types, you dont want to make it too wonky. you can avoid that by drawing the head before the hair and then make sure you keep it somewhat proportional unless you have one of those styles that big fluffy hair is absolutely required

be careful if your doing shading, it´s easy to put the wrong shadows in the wrong spot along with highlights, it´s also really easy to over do it and get a really muggy result, look at references, and draw a little lamp in the corner to know where your light source is