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"Anything in evolution can be said to have a purpose" sounds a bit teleological to me, dear John. However, without drifting into philosophy (considering intelligent design etc.), a certain degree of mutualism between Salix and Egle cannot be denied. As the flies feed on the male as well as the female inflorescences and oviposit in the latter, cross-pollination of the dioecious plants is facilitated. There is a similar correlation between the hermaphroditic Trollius europaeus and Chiastochaeta. However, there is no total interdependency; the flowers may well set seed through other visitors or (Trollius) by selfing.
The hairs of Egle are not exceptionally long, but - and this is interesting! - their diameter seems to fit well into the colpi (furrows) of the pollen grains of Salix, so that the grains "grip" the hairs. Look at the nice pictures in https://www.bayce...H_2016.pdf p. 40. So colpate grains obviously are better adapted to this kind of transport than are porate grains. Floral biology is such a fascinating field; regarding this fact I pity that I am retired...