The possibility to use simulated devices, so that you can design software without having the hardware available is very nice. Sometimes, specific signal profiles are needed. It can be tested using additional software, but you really have to change the software (add test-software in the code). In some cases this is not wanted.
Therefore adding in DAQmx a possibility to force specific output can be very helpful. Different possibilities exist (TDMS-files, LlabVIEW(/compiled LabVIEW)-code, ...). Different people will want different possibilities, but some extra possibilities are useful.