I've been getting my gas from there for years - never had to buy any beer from them.
Maybe it's easier once you have a gas bottle to swap. I use the Robin Hood depot - near the Halfway House pub on Wakefield Road.
'Racking' is just the process of transferring from one vessel to another. IMO, there are no real benefits to transferring to a secondary unless you are planning to bulk mature for an extended period of time. Others may have a different opinion.