Why do birds sit together on electrical wires?

They are social creatures and likely to hang out where everyone else does - like people! They tend to gather in flocks too. If it's near intersections my guess would be that's because it's between two clear fly zones, where there are not tall trees or buildings to get in their way as they fly. They like to stay relatively close to the ground for warmth, water, food, action...and wires are everywhere so they are used to hanging out there.