I've only recently become interested in international football.
I notice that, what seems like is all of the times, the goalkeeper will go to the ground to collect a rolling ball.
Is this a rule? Or maybe a best practice?
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Not a rule in any way, the goalkeeper can pick the ball up just about any way they like (if they're in their own penalty area, obviously).
What they're trying to avoid here is any chance of a bobble or just a slight misjudgment on their part which results in them missing the ball - get as much of your body behind the ball as you can, and then you minimize the chance of something going horribly wrong.
Goalkeepers will go the ground for one of two reasons,
to avoid a spill that an opponent can challenge for, or a minor error leading to the ball going past them
they doubt they will collect it before the opponent touches it
In the first case, the ball is made more secure by having more of the arm and legs and torso behind it. Such a body position requires being low to the ground, in effect lying on or behind the ball, as tends to be the final resting position.
In the second case, using the "long barrier" prevents a slight touch from causing the ball to be put past the goalkeeper or causing them to spill it more easily.
It is also more likely when the goalkeeper is collecting the ball at high speed. Bending over to pick up the ball is relatively easy; doing so at a fast run is not straightforward, at least in a way that allows the goalkeeper to continue running much further.