RYANAIR RECEIPT, what is it good for?

I just received two emails each with a RYANAIR RECEIPT for one of the two flights. Not that I wouldn´t know what a receipt is but I have never received one from Ryanair before so that makes me a little curious. Also, I have other flights, earlier than this one and didn´t receive any receipt.

One thought: can it be because I am flying from Madrid to Palma de Mallorca and back?

Thanks

yes, because it's a domestic flight was the same on german domestic flights, where i was wondering as well!

good, thanks for the response