My understanding is that it is to hold the stirrup leather in place if you lose your lower leg, for example while jumping.
Whilst it may not be the safest in every situation, the last thing you want as you are sailing over a jump and have lost your lower leg (lets face it, happens to the best of us) is for the stirrup leather to slide off. Then you're really in trouble.
I tried to find a picture and found one that sort of illustrates what I am saying: lower leg jumping.jpg
Although the rider is quite solid still in this pic, if they slid their leg just a little further back and still had weight in the stirrup, they would be at risk of sliding the leather right out of the holder and attempting to land with one or no stirrups. If it is locked you have that bit of protection from losing a leather mid air if you fumble a jump.
So there are risks in both scenarios and it just depends on the situation and the rider, as everything!