It is dangerous to have knowledge reside in only one person’s head.
Suppose Trixie is the only person who knows how to work on your web site. No one else even knows how to log in, let alone change the site. Trixie is hit by a meteor. The next day your web store breaks and won’t accept orders. The oxygen to your business is now cut off. Have fun finding a web programmer who will drop everything to rush over and figure out Trixie’s home grown web store. Maybe Trixie wasn’t hit by a meteor. She might just be camping for a week. Either way, you are losing massive amounts of money, your customers think you are incompetent (they are right), and no one is getting work done as you all run around screaming.
Another danger of only one person knowing how part of your business works is that they have you by the short hairs. If Trixie is evil then she will start arriving late and leaving early. Maybe she isn’t evil but just slack, same result. What are you going to do about it? Perhaps not much because you know she could destroy your web store.
Create a culture of shared knowledge as early as possible. Being able to relax on vacation is enough reason for most people to get on board. For the others, kick their butts. An atmosphere of relaxed information sharing is worth fighting for.