Often times getting to Half Guard first gives you a goal, or starting point, to work from. Consequently, passing from the Half Guard can be thought of as being closer to accomplishing this major goal than trying to pass from full Guard.
There’s a concept that is useful for getting yourself into the top Half Guard game in which Jason Scully teaches called the Step-Step Concept. It involves breaking open your opponent’s legs from a standing Guard, stepping back with your leg, and then immediately stepping forward with the very same leg. Your trailing leg becomes susceptible to being entangled by your opponent. However, the leg which stepped back then forward has already begun the pass, putting you a step ahead of the opponent who is trying desperately to hold the Half Guard position. Interestingly, the X-Pass uses a kick-back movement followed by a full step forward that basically accomplishes the same mission as the Step-Step Concept.
Obviously, getting the full Guard pass is preferable. But when you're up against someone of equal or better skill than yourself, perhaps small victories are more attainable. Going from a Closed or Open Guard into a Half Guard position can be that small victory. So if you're having trouble passing the Guard against a skilled opponent, try forcing them to put you in the top Half Guard and begin your game plan from there. You'll already be a step ahead of them, and you'll get a chance to work all those cool top Half Guard techniques.
Learn. Drill. Roll. TRANSFORM!