It’s always nice to have a fallback option. Of course everyone has a plan that they want to stick to, but when that plan doesn’t pan out, a safety net can quickly become a lifesaver.
The same rule applies to the crease in hockey. An ideal setup for any hockey team is to have a lockdown number one goaltender in his prime as the starter and a young but skilled goalie to back him up and eventually assume the role of starter.
There can be different variations to this setup that also prove to be successful, but this is typically the most efficient way to build a crease.