The development and necessity of the frequency settings in astrometric and geodetic Very Long Baseline Interferometry (VLBI) observations are analyzed, especially on the advantages and disadvantages of observations at Ka-band. Compared to X-band, the advantages at Ka-band include more compact extragalactic sources, relatively smaller core shift effect, as well as efficiency to improve the precision in the realization of radio celestial reference frame and in the linkage to the quasi-inertia optical frame of Gaia project. Advantages in terms of deep space exploration are high in telemetry data rate, high in VLBI tracking precision, and helpful to mitigate the detrimental effects of ionosphere and solar plasma on the observed time delay. Disadvantages include relatively weak extragalactic sources, high precision requirement of reflector and antenna pointing, and susceptible to atmospheric absorption and emission effects. But the loss in system sensitivity due to these factors could be mostly compensated by the rapid development in high data rate sampling and recording technology. Finally in accordance with the current status of VLBI technique in China some specific recommendations for further development are recommended for criticism by readers.