Jackdaws do not migrate, however there maybe some slight Southerly movement from those that breed in the North. As for why they flock together it is the same as for many other species i.e finches, thrushes and tits it is easier to forage in a flock as there are many more eyes, once the breeding season is done with territories are not so important for the birds that tend to flock in winter as they will start setting them up in the spring.
Species like the robin however will defend their territory all year round which is why you never see them flock.
Actually if you observe carefully you will see jackdaws flocking all year round as like other corvids they are an extremely gregarious species, hope this helps.