The Mishna (Beitzah 5:1) in today’s daf (Beitzah 35b) informs us that “We may put a vessel under dripping water on Shabbat [and then move that vessel as and when the vessel is filled]”.
Significantly, the halachic concern which the Mishna seeks to clarify relates to the rain or drain water that has dripped into the vessel which may have been considered ‘mukzeh’. However, as the Gemara (see 36a) and later commentaries clarify, since the water could be used for drinking or bathing, or even to water animals, it is not considered ‘mukzeh’. Still, if this is not the case, and if the water is particularly dirty, one should not move the vessel filled with dirty water on Shabbat.
Of course, the ideal would be to have a house without a dripping roof or drain. And if it does drip, the ideal would be that it only drips the smallest amounts of clean water. But the way of things is that sometimes roofs leak, and sometimes they bring the types of water that we’d rather avoid.
And when that occurs what do we do? We put a vessel under the drip, knowing – on occasion – that the vessel will be filled with dirty water, and knowing – on occasion – that we cannot move the vessel for a while. Yet also knowing that while we can’t always control if, where, and when water will drip, we can be there to catch the water when it falls.