They can be used as 'feet' to raise other pots off the ground - may need to put a couple inside each other for strength if pot is heavy.
You can scrunch them up in the base of big pots to reduce the amount of compost needed - if planting shallow-rooting, cascading plants in a tall pot.
Some garden centres have collection points for recycling - Dobbies does near here.
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Where did 'please' and 'thank you' go?
Does anybody work out with kettlebells ?