The 1900D isn't a good example of tailpane 'winglets'; they're aerodynamic hacks to give more keel area without changing the vertical stab from that of the King Air 350.
Beech looked at installing a new stab but it would have triggered recertification.
So, they went to put big vertical plates on the tips of the tailplane to compensate but found that they overloaded the tail mounting structure. So part of the additional keelage was added as those big ventral strakes and what was left sticking-down from the tailplane was the most that could be accommodated without structural redesign.
Same story for the 'smurfs' on the flanks ahead of the tailplane; a new tailplane had originally been intended but was abandoned.