Does grid drawing help with drawing from imagination?
Perhaps the main benefit of grid drawing is the accuracy it lets you achieve, but it also helps train your eye to see what's actually there, as opposed to what your brain thinks is there. 

When you analyze the contents of each individual square, you're forced to focus more on the lines and spaces that make up the subject. It helps guides you through measuring those angles and proportions, like I mention in this answer about portrait drawing. Instead of comparing lines simply to themselves, you can compare lengths and angles to your clear, consistent grid lines. 

It also helps you to get a better understanding of scale and proportion, especially when dealing with perspective and foreshortening. Your brain may know that a leg is longer than an arm, but in 2D representation this is not always the case. 

So, while drawing from imagination you may think "I know the leg will be shorter than usual at this angle", you may over/under-estimate how much to adjust the limb for foreshortening. However, drawing from the grid will give you experience to help you make a more accurate decision. 

These skills will be applicable to any drawing you're doing from reference, but also help train your eye to see what's right/wrong about things you draw from imagination. It's just another way to build up your ability to critically analyze what's actually there. 

However, it's just one component that will help you in your drawing from imagination. For figure drawing in particular, it's helpful to learn how to construct figures using some type of method, such as blocks-and-cylinders. Drawing from life, in general, is going to give you a better understanding how the world actually looks and works. For instance, you may be able to construct great mannequins, but without having a good mental repository of real facial features and body types, you'll likely have very stale figures.

Since your focus is drawing people and poses from imagination, I personally would pursue other forms of practice over grid drawing. If I were to compare drawing by grid to drawing from materials by artistic anatomy masters such Jenó Barcsay and Stephen Rogers Peck, I'd pick artistic anatomy as a clear winner.