Static balancing means balancing in a single plane. Dynamic means balancing in more than one plane. Static balancing *can* be done (but is rarely) without spinning the object in question, dynamic balancing can only be done by spinning the part.
Anything that's essentially flat only needs a static balance - e.g. imagine balancing a CD, if you find something which fits the hole and mount it with the CD vertical (your 'axle' horizontal) in frictionless bearings, then it will rotate and stop with the the heaviest point at to bottom.
Now imagine something with significant length along it's rotational axis, e.g. a propshaft. You could put opposing weights (180 degrees opposite) on at opposite ends and it would remain statically balanced, but it's dynamic balance (i.e. when spinning) would be appalling.
For wheels, the wider the wheel the more essential it is to dynamically balance it.
Mike