Level by level multiclassing is a very useful tool for designers. It allows them to make fewer base class while having a larger list of virtual base classes.
I think it is a good idea for optional content. It does not need to be used, but it allows players with non standard characters to play D&D instead of leaving to find another RPG that can handle their character concepts.
For example, in the absence of a lycanthrope PC species, you could have a 5E Totem Barbarian / Moon Druid to represent a warrior that is fighting to hold back the beast within. Start the fight as a calm fighter. On round 2 activate Rage as combat decreases your efforts to hold back the beast. On round 3-4 Wildshape into the beast within.