A suction device can only pull one atmosphere, which is about 14.7 PSI. No more than that.
A pressure device can push with more than 14.7 PSI, and therefore can overcome system resistance more easily. The effect is that the volume bled will be greater per unit time, and the job can be done faster.
Additionally, if the syringe drawing the vacuum is limited in volume, compared to the volume of the hydraulic system, some of the vacuum volume which will be used to bleed the system may be taken up in the limited, but not zero, elastic properties of hoses and other components.
Furthermore, a higher purge rate, obtainable with a pressure feed bleeder, will tend to evacuate air and other contaminants, due to turbulent flow, and other effects of a higher flow (flux) rate.
A very low flow rate for bleeding would be to simply open the bleeder on the brake, at the low point in the system. This tends to not work well, because some air and contaminants may not get flushed with a low flow.
In summary, using a pressure bleeder helps better purge the contaminants and has the added benefit of making the job go faster.