There are many reasons why you would use a mounting block to get on your horse. Other than the fact it's easier for the rider to get on, the biggest reason to use one is that it is far easier on the horse’s back.
There is no pulling, possibly twisting the saddle and possibly pulling the horse off balance. I know there have been times when I’ve hoisted myself up in the saddle when my poor horse has had to take a sideways step to compensate for the pull I’ve exerted. Also, when you are stepping into the saddle from a mounting block it is easier to settle into the saddle without dropping down onto the horse’s back with any type of force.
The only disadvantage to using a mounting block consistently is that if you are out on the trail and have to get off for any reason, you cannot rely on having a handy “step” to get back up. There are some products available to take along with you in your saddlebag - such as a mounting stirrup from Chick Saddlery or Schneiders' Easy Mount Step Stool - but unless you are riding in a completely flat field, you can usually find a rock or a stump to help you get back on any horse.
It doesn’t take long to teach a horse to stand at a mounting block, especially if your horse has been taught good ground manners. Lead your horse to the block and have him stand alongside it while you step up onto it. Once he is standing straight and accepts your movement alongside him, then you can step into the stirrup and onto your horse.
A word of caution – it is wise to use a solid mounting block that is built specifically for that use. A plastic stool with open spaces could be a recipe for disaster if your horse takes a sideways step and plants a foot into the stool (I know this from experience!). I also know of someone who used a 5-gallon bucket which tipped while she was standing on it and she ended up with a broken leg!
There are many different models of mounting blocks, such as 2-step, 3-step or even stools. It's a matter of choosing which one is easiest for you and your horse.