Why did WW2 soldiers have fish net on their helmets?

To hold camouflage, take a few fresh-cut twigs from local flora and insert it in the netting – breaks up the distinctive size and shape of a human head. Even on its own it softens the edges of the helmet making it more difficult to pick out.

As the first two pointed out... to hold improvised camouflage (twigs, leaves, grass) to break up the outline.

DO NOTE though that NOT all soldiers would have this netting... it would be limited to front-line infantry and paratroops.

to hold improvised camouflage items on them