I guess he doesn't want those cute little birdies to eat his fruit.
That's to keep the pests off, possibly moths, possibly birds. Sorry for such a boring response.J
As Kitty said, this is a tree helmet. People are too accepting of moths and birds with anti-arboral attitudes, so they put too much responsibility on the trees for their own not really concrete theoretical safety. It is quite similar to the situation with bike helmets: Way more trees in Denmark have these things.
I'm so glad nobody suggested anything along the lines of "tweehuizig" (dioecious) :-)Yet.
Hmmmm... keeping butterflies (and thus their caterpillars) at bay; protecting young leaf-buds from strong wind or sweltering sun?L