It also shows how many people are affected by the buiff in the middle of your screen.
The speed is obv handy for getting back from a wipe, or getting away from an ulti, also remember the E button amplifys the effects so an additional healing/speed buff.
One of my fave things with lucio is the alternate fire knockback, knocking enemies off the edge of a map is so satisfying
My favorite thing with lucio is one on one with mei and when she goes low health she iceblocks so I just stand there and wait and when she unblocks I knock her back dealing quite the dog and shooting her.. she's dead xD