Because if your reserve 'keeper is 18 years old your 1st choice 'keeper has less incentive to be at the top of his game week in, week out. Having a two keepers of equal ability brings out the best in them.
C. Smith and Howard had good seasons when they were battling it out to be #1. Gallacher was excellent in his 1st season when he was up against Howard, and was rubbish in his 2nd season when DL made it clear Samson was in reserve. Sammy had a good season up against G. Smith, and hasn't been as good this season when the other 'keeper, has been unavailable for various reasons.
Just my hypothesis, mind.