Dutch Referee Blog - Always put an a4 size paper in your referee bag

What’s in the referee bag you take with you to your matches? Just add some papers on a4 size …
… because sometimes the substitution board doesn’t work due to heavy rainfall, like here in the Chinese Super League match between Hangzhou Greentown and Beijing Guoan.

 

[Reffariiii]

So, in the rare, but granted, possible event of this happening, how is a4 paper going to make a better solutions, as I was always under the impression that paper and rain don't work togeather...