Using a mounting block is a great way to show you care for your horse and value your equipment.
The lower you are in relation to the stirrup bar, the more sideways pull you put on your horse. This means the horse has to brace their muscles on one side as you get on. This can lead to musculo-skeletal problems and/or asymmetries that can impact your horse's performance under saddle. The same sideways pulling forces can also twist your saddle and stretch stirrup leathers over time.
A mounting block is also great for training! It gives you a reason and target for making your horse stand still whilst you mount.
It is important to use a safe mounting block with a wide, non-slip and durable surface area.
Milk crates should never be used as they are not designed for this purpose - many riders (and horses) have had their leg go through a crate.
Here at Aussie Saddlery we use and reccomend the Mount Ease mounting blocks.