It's a mystery, unless crows or God could tell us hehe :)

It depends on several factors: time of year, age of the birds, habitat suitability. A lot of information is transferred between crows when they flock such as areas to feed, rest, and places to avoid. Outside of the breeding season, crows and other birds tend to flock also to increase survivorship against predators.

It's speculative for the most part. Thermal regulation, very social nature, increase foraging success. I am not all sure about an anti-predatory function since flocking makes them more vulnerable to the nocturnal predators of consequence, at least in the United States (viz., Great Horned Owl). There is plenty of literature on this subject.

Hi.

I agree with the answers of Kelly and Craig. In general, corvids are very social and gather in flocks, and some (or many) of them are colonial during the breeding season. But there are species very territorial and solitary when they have nests, like Common or Northern Raven (Corvus corax), which later, during the summer and autumn-winter, form big groups.

Best regards.