Changing focus may be a problem for some children.

It also relates to eye comfort when viewing an object from a distance. There is an interesting comment on that in the Notes below.

I experienced a similar problem, to my surprise, when I tried to instruct my sons in the proper use of fire arms (in this case a beebee gun).

Although I had never had problems with near/far focusing, this was one of those little surprises that crop-up with the aging process.

I could see the bead on the end of the gun barrel clearly, but when I switched to look at the target, it took so long to focus on the target that I lost all sight and control of the gun barrel direction. And when I switched back to the gun barrel, it took so long to focus on it that Iost sight of the direction of the target.