Yes, it's getting rid of the steam pressure build up so it's easier to get going, once up to a speed, they can close that valve and the pressure will help. Admittedly I think some of it will be for show, but they absolutely do need to let out some steam so as its not trying to get going under load.
I'm a total layman, is that amount of steam blasting out of the front end really and absolutely necessary.
Yes, it's getting rid of the steam pressure build up so it's easier to get going, once up to a speed, they can close that valve and the pressure will help. Admittedly I think some of it will be for show, but they absolutely do need to let out some steam so as its not trying to get going under load.