Question about normal draft

Curious about the design decision behind requiring locking in your champion selection. I have been away from league for about 6 months and am getting back into it with some friends I forgot to lock in two times and now I either force them to wait 15 minutes for me or I have to sit out a game. I had chosen a champion I just got distracted by choosing runes and masteries.

So I want to know what the reasoning behind the decision to require locking in is.