It is very easy to add or move fields by just dragging them. Most(if not all) of the popular plugins offer you this though.
Once you want to add more fields to a row is when it become interesting, many plugins doesn't support this or you need to modify the settings of each field which is tedious or complex. With Smart Forms you just keep dragging.
Do you want to resize a field? You just need to drag!
There are also fields that let you drag inside them to create groups.
Lastly, you can also resize the form designer by dragging the border.