While mounting blocks are absolute musts for some of us (myself included considering I'm only 5'2 and two of my ponies are 16hh), they can be very beneficial for everyone looking to mount up no matter what their horse's size or their physical ability.
#1) Using a mounting block causes less wear and tear on our tack. When I was in pony club I was taught to rotate my english stirrup leathers from time to time because the left leather can become stretched from mounting. The same is true for western tack. I have an old saddle I use for starting young horses and the left stirrup is one half notch longer than the right from 40+ years of mounting and dismounting. Some saddles even develop slight twists in their trees from mounting.
#2) Using a mounting block makes mounting more pleasant for your horse. Let's admit it. Riders come in all shapes and sizes (as they should!) and most of us are a little less than elegant when we attempt to swing up into the saddle. Instead of clamouring and scaling the leather landscape of your tack the next time you mount up, use a block. No need to worry about poking your pony in the ribs with your toe or twisting your saddle until it pinches your horse and causes them to wince at the thought of your next ride.
#3) Using a block is a great way to reinforce good behaviour and maintain your horse's respect. It forces us to use the groundwork tools we worked so hard to develop early on in a horse's training. You know - the ones we often take for granted and then wonder why our horses aren't behaving the way they should anymore. Each time I line Quiggly up at the block I can quickly assess his attitude and offer a quick tune up. This is key to our success considering he tends to challenge my leadership fairly frequently. And it only takes 30 seconds!