Clay is sticky, holds water for long periods, and accumulates in wetlands each year. Clay also breaks up into dry blocks in the summer making it easy to collect!
Wetlands in the Gangetic flood plains have a lot of silt and clay in the water that are continually churning due to water flows and human activity. The finer silt settles down, and on top of it the coarser clay is layered. This can "kill" the wetlands - making them easier to convert to croplands, filling up the depression, and holding up moisture that would otherwise have likely percolated to the ground water store.
The ancient practice of removing clay during summer in Uttar Pradesh by the people keeps wetlands from dying. Block by pain-staking block of dried clay is broken off, and ferried by hand to the road where a bicycle awaits.
Entire villages stockpile the useful clay. They are used to repair damaged walls of mud-huts, fill up leaks or as a natural cement for brick-houses, to make clay-bricks, or to make dykes of flooded paddy fields. Villages therefore benefit from maintaining village ponds, and collecting clay is often a community activity. The following year(s), the process of soil accumulation begins all over again in a now-deepened wetland. Unwittingly, natural habitat for a variety of birds and flora is maintained.
Newer "technology" has allowed many people to build houses that do not need annual maintenance. In such areas, village ponds like the one above likely require the clay to be removed, but is not paid attention to. The landscape ends up losing precious wetland habitat, and the people lose opportunities to work together.
(Thanks to Shyamal for a lesson in Soil 101. All photographs taken on 30 Apr 2010 at Rae Bareli.)
Post-script: Illegal usurping of community wetlands can lead to unfortunate disputes - for a recent story on a shoot-out due to clay removal, read this.