We Say:
We recommend that you always carry the Deuter sun roof and rain cover when you are taking your child out in the carrier.

Obviously if the rain comes in you can keep them drier but from personal experience the sun roof can be invaluable on a hot day to help prevent little ones catching the sun ....and they can't throw it off like a hat.  

The sun cover is made from a close knit mesh material and the rain cover is stored in a zip compartment at that back.  If the weather closes in you whip out the rain cover and attach it over the top of the sun cover (as shown in the photo).