Tail closure
I think the question was why all the vortex generators on the tail near the APU inlet. The 737 fuselage cross-section is the same as the original 707, a much LONGER A/C. As explained by my now long retired Boeing engineer father-in-law, the VG's were an afterthought, added to help smooth out the slipstream around the tail. Nothing to do with the APU. The 737 was meant to be an .80M aircraft but when it first hit the market, .72M was about all it could do before it started to buffet. Wind tunnel tests confirmed the buffeting above .72M was caused by the abrupt change of airflow at the tail. One solution was to add a stern fairing, but it would have to extend far beyond the tail, looked hideous and Boeing thought it would negatively affect sales. Hence the vortex generators. Note: a certain number of them must be present per the MEL.