Generally it's to insure all my shamans are in range (Max range). I typically will when unsure of the range, start a test cast, then stop then readjust distance. Especially on bosses. I hate to not be at max casting range, so I'll try to get every inch of distance I can. Once I see all shamans are in range i'll cast away.

Sometimes I'll notice something I missed, and jump cancel to change targets / avoid pats, usually it's a range thing.