Each time you hit b in the combo you can also hold side, up, or down to change what the next move in the combo is. It's probably best to just stick with hold one direction though since they seem to flow better that way. All three major combo strings are useful in different ways - I find that the side combo is good for covering ground and approaching into rollers, the up combo launches higher and is better against spot dodges, and the down combo is safer at landing on shields.
Most importantly, if you start a side b combo midair and land, you can finish your combo with no ending lag, making it a strong air to ground approach option.