leaves needle-like, but with a groove on the undersurface containing most of the stomata (leaf "vents") and filled with hair.

Two theories:
OLD: leaves are rolled up to expose upper surface with only a few stomata and function to reduce waterloss in a dry summer environment;
NEW: in summer plants dormant: leaves irrelevant. Ericoid leaves wick water from the mist at high altitudes, but more importantly rapidly get rid of ("waste") water in winter (NB: opposite of above!!) perhaps in order to keep roots aerated in otherwise waterlogged conditions (but mycorhizal symbionts get most of the nutrients for the plants and can cope with waterlogging).

I'll try and find the paper for the latter theory, but obviously I have over-simplified. Both apply: when dry the leaves roll tight closing the groove and saving water, but when wet, the groove opens and water can maximally get out (or in - in mists).