oh...

"You might think that the bigger gaps where soil meets bits of broken crockery would allow more water to filter through. But this turns out not to be the case, some argue. Guy Barter, chief horticultural adviser at the RHS, says a crock is actually likely to worsen drainage by creating a block. It's better to have a layer of sand underneath soil that will allow water to drain into it and later be sucked up by roots if needed. The only minor points in favour of crocks are that they might block drainage in hot weather, and also save money on compost. In most cases, people would be better off omitting them."

Alan Titchmarsh - Crocks

"Broken pieces of clay pot, which are used to cover the holes in the bottom of pots. The crocks help to stop compost blocking the holes and improve drainage. Stones, coarse gravel or large fragments of polystyrene are useful alternatives."
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EDIT: from my friend Alexandra: The Myth of Drainage Material in Container Plantings: