Recent work on comb vibration suggest that one purpose of the vibration is to attract followers

> Among honeybee species, dwarf bees, Apis florae, conduct dance communication on a single comb in the open, while rock bees, Apis laboriosa, always dance silently. It has therefore been suggested that these bees might be able to utilize visual information conveyed by their dance behavior. Hive bees, Apis mellifera, however, conduct dances with rhythmic sounds in a dark hive. Studies of morphological and molecular characteristics have suggested that dwarf bees and rock bees diverged from hive bees such as the Western honeybee. The ability to detect the rhythmic components of sound might have evolved after this divergence.

> Our study suggests that honeybees conduct dances that produce rhythmic sounds to attract followers in a dark and noisy hive. The utilization of rhythmic components presumably evolved to allow bees to determine whether a target with information or the information itself could be detected in a dynamic environment. 

Hasegawa Y, Ikeno H (2011) How Do Honeybees Attract Nestmates Using Waggle Dances in Dark and Noisy Hives? PLoS ONE 6(5): e19619

Other work suggests an additional role as an indicator of the quality of the food source

> In the present study, we asked whether these vibrations are a reliable estimator of the excitement of the dancer when food profitability changes in terms of both sugar concentration and solution flow rate. The probability of producing thoracic vibrations as well as several features related to their intensity during the waggle phase (pulse duration, velocity amplitude, duty cycle) increased with both these profitability variables. The number of vibratory pulses, however, was independent of sugar concentration and reward rate exploited. 

> Thus, pulse number could indeed be used by dance followers as reliable information about food source distance, as suggested in previous studies. The variability of the dancer's thoracic vibrations in relation to changes in food profitability suggests their role as an indicator of the recruiter's motivational state. Hence, the vibrations could make an important contribution to forager reactivation and, consequently, to the organisation of collective foraging processes in honey bees.

Hrncir, M., Maia-Silva, C., Mc Cabe, S. I., & Farina, W. M. (2011). The recruiter's excitement–features of thoracic vibrations during the honey bee's waggle dance related to food source profitability. The Journal of Experimental Biology, 214(23), 4055-4064.

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