I don't understand what you mean by "added two different speed of velocity" -- the linked video doesn't show anything.
If you change MIDI note Velocities for a recorded Software Instrument Region that will only have an effect if the Instrument responds noticeably to [sounds different at] different Velocities; if it doesn't -- like an organ or some synth sounds -- then editing Velocities won't have any effect.
No, MIDI Velocity relates to the musical dynamics. As I said the effect will depend on the particular sound/Instrument. If you don't have a specific reason to change it, then don't. If you're trying to change the level of a particular Instrument track in playback use the volume slider in the Track header.