With a one column form you have all your fields stacked on top of each other, this is great for when you have a few fields and want to have it sitting beside content!
Better for situations where you might need to ask for more information - keeps the overall form height shorter while being abel to ask for more information
More complex forms where you might want to group or separate fields. Things you might want to group are: First Name / Last name OR location data, like Country, State, and City.
