In some forms groups of unrelated species might form, in which interaction between members of the aggregation might be minimal; for example herds of grazing zebra and antelopes might combine, both the better to observe the approach of predators, and to improve the odds of escape in the event of attack by predators.
Sometimes there might be some interaction, such as mixed flocks of birds that observe each other's foraging behaviour in searching for food.
Another is when aposematic larvae of certain insects such as some Lepidoptera and Hemiptera form a flock that feed together and may migrate in "processions" until they are mature after which some kinds disperse.
The range of functions is very wide in detail, but among the more important classes of function are security against predators, success in food location, wide range of mate choice, with concomitant increase of outbreeding opportunities, location with other members of the same species (sometimes adherence to separate communities can almost amount to parapatric residence when say, different communities of rats or chimpanzees have violent mutual antipathy).
There also are various forms of educational function, such as in some species where the young must learn the correct mate recognition skills, and in highly intelligent species such as crows and elephants, must learn the necessary social skills and the necessary traditional foraging techniques in their region.