I'm an SLP and I see this in a lot of the speakers I work with. Some people seem to talk fast because if they "get a good start" and just "launch" into it, they hope things will turn out OK. The trouble is that this doesn't really put you in control. I liken it to a runaway train on the tracks-- it may be going fast and smooth, but if it hits one snag it easily becomes derailed and it's harder to get back on track. Versus if you have someone driving the train, it might go a little slower and steadier, but you're much better able to anticipate bumps and navigate through them. And you have much more mental/psychological control as the driver, instead of just "hanging on" and hoping the train goes where you want it to!