Well, there are several reasons to use the neck lanyard:
1. Keeps your ticket more readily available than in your wallet, purse, or fanny pack.
2. Keeps the pass away from other objects like cell phones or other cards which might demagnetize it.
3. Keeps the pass dry and flat - ever get your wallet wet from sitting on a wet ride, or have a card get crinkled from being in your wallet or fanny pack?
4. Provides an additional, convenient, safe place to keep FastPass slips.