The L-39 is probably a civil machine on loan to the USAF
from the National Test Pilots School (NTPS). That organization uses all sorts of machines to privately explore the envelope of aviation. Examples include; Saab J-35, MB
-326, Mig-15UTI, PC
The CASA CN-235 is in use by the CIA
/Special Forces. The USAF
operated similar "interdiction" aircraft in Panama (C-212). These airframes fill a small niche in transport that a C-130 or MH
-53 cannot answer.
has many different types of aircraft for different purposes.