A quick Google reveals this:

Homemade Substitutes for Toilet Paper

I have no idea why grass and leaves are added to the water before boiling.

What is interesting is that some other alternatives are also mentioned, including some non-poisonous and non-irritant leaves and corn husks.

Which reminds me of a little ditty my Dad used to say:

In days of old when knights were bold and paper wasn't invented,
They wiped their bots on tops of grass and walked away contented.