What are the evolutionary advantages of male beard growth?

There is a strong genetic component of variation among human males in patterns of facial hair growth and ultimately this is what allows evolution. However, by
"evolutionary causes" I'm guessing you really mean what is the adaptive significance of beard growth, i.e. why do men grow beards when women don't.

Assuming that this difference between the sexes has been selected for, then the most likely explanation would be some form of signalling - either to attract women, or possibly to deter rival males (or maybe a bit of both). I've pasted an abstract from a study that seems to support the male-male signalling hypothesis below. Given how much cultural/social variation there is in people's views on facial hair (both geographically, and through time) I think it is alway difficult to test these hypotheses out on contemporary adults without risk of bias. For instance if you tested in the 70's you might find that on average women found men with big moustaches more attractive, but the same test today would likely produce the opposite result!

Its also important to realise that selection is not the only player when in comes to evolutionary processes. It could be that beards in men are simply a by-product of different hormone levels in men as opposed to women, and that while hormones are important for fitness, beards themselves have little adaptive significance.

Title: The evolutionary significance and social perception of male pattern baldness and facial hair
Author(s): Muscarella, F; Cunningham, MR
Source: ETHOLOGY AND SOCIOBIOLOGY   Volume: 17   Issue: 2   Pages: 99-117   Published: 1996

Abstract: Both male facial hair and male pattern baldness are genetically based, suggesting that they contributed to fitness. The multiple fitness model provides an evolutionary interpretation of the social perception of male pattern baldness and beardedness in terms of the multidimensional meaning of physical maturational stages. Male facial beardedness is associated with the sexual maturation stage and is hypothesized to signal aggressive dominance. Male pattern baldness, by contrast, is associated with the next stage of physical maturation, termed senescence. Pattern baldness may signal social maturity, a nonthreatening form of dominance associated with wisdom and nurturance. We tested these hypotheses on social perceptions using manipulated male facial stimuli. We presented faces with three levels of cranial hair, including full, receding, and bald, and two levels of facial hair, beard with moustache and clean shaven. Consistent with the model, a decrease in the amount of cranial hair was associated with increased perceptions of social maturity, appeasement, and age, and decreased perceptions of attractiveness and aggressiveness. Targets with facial hair were perceived as more aggressive, less appeasing, less attractive, older, and lower on social maturity than clean shaven faces.