Shin reinforcement adds extra-durable knitting over the front of your shin, where your boot tongue presses and rubs throughout the day. It's not essential for everyone, but it makes a real difference if you ski aggressively or have heavy boot contact.
The main benefit is that it keeps the shin cushioning from packing out prematurely. Without reinforcement, the padded area over your shin can compress and thin after repeated use, leaving you with less protection against boot pressure. Reinforced construction helps that cushioning last longer.
Shin reinforcement is especially worth seeking out if you're prone to "shin bang" — that bruised, painful feeling from leaning hard into your boots all day. Park skiers, racers, and anyone who drives their shins into the boot tongue will notice the difference. For casual resort skiers who don't experience shin discomfort, heel and toe reinforcement matter more.