For transport aircraft with common high-load situation, it needs to pitch high (high AOAs) to gain necessary lift.  But when doing so at high AOAs, the vertical tail will be seriously influenced by the downwash and turbulence from the lifting body (in this case, the fuselage) and hence the vertical tail's effectiveness in controlling the yaw would be drastically reduced!  In such kind of situation, the two small winlets at the end of the horizontal tails will be outside of the fuselage turbulence field and hence can compensate for the lost yaw stability and effectiveness of the main vertical tail.  

In the case of Y-9, using the differentials in the two low horizontal tails cannot provide enough additional yaw control compensations in a turbulent field as when with low AOAs (This is unlike the case of a transport aircraft designed with a high-T tail).  The unwanted roll from the horizontal tail differentials can be offset by slightly adjusting the aileron positions on the main wing.  However, for a transport aircraft with a high-T tail design, the upper part of the vertical tail and the two horizontal tails would be outside of the downwash and the turbulence field even at high AOAs, and hence can retain its yaw control capability by using the partially-effective vertical tail outside of the turbulent field, plus augmenting with the differentials of the two horizontal tails.