According to G. M. Barker's The biology of terrestrial molluscs (CABI, 2001, 558 pp.):
"Periostracal hair-like or scale-like processes are not uncommon in a number of stylommatophoran families. A number of hypotheses have been advanced for the functional role of these periostracal processes, including: (i) ‘to repel moist particles’ (Solem, 1974) and prevent wet leaves from adhering to the shell; (ii) to defend against predators (Webb, 1950); and (iii) to camouflage the shell by trapping soil and debris (Pilsbry, 1940). These hypotheses are yet to be tested."
I also found this open-access article:
Pfenninger, M., M. Hrabáková, D. Steinke & A. Dčpraz. Why do snails have hairs? A Bayesian inference of character evolution. BMC Evolutionary Biology 2005, 5:59. DOI: 10.1186/1471-2148-5-59