Basically what it says on the tin. Giving Tiny and Bigs an extra function, by allowing you to pay pebbles in exchange for water and compost.
Either purchasing a set amount (e.g. 50 water and 50 compost for 5k pebbles, which SHOULD allow you to go past your 'natural limit') or purchasing to fill it up to the maximum (so say I have room for 33 more water and compost, I could purchase that for 3.3k pebbles)
Numbers are purely meant as an example.