Boiler Valve Corrosion
1) Is there any purpose for this valve other than for removing the pressure gauge?


NO, the valve is only there for isolation of the gauge.
Gauges like that have a high failure rate (for various reasons) and often need to be replaced "on the run"; that is, while the boiler is still in operation. 


  2) Is this a huge/expensive/messy accident waiting to happen if I don't do something about it?


The construction of that type of gauge uses a Bourdon Tube, in which the process fluid (e.g. hot water, steam) is actually present inside the gauge. Failure of the gauge could indeed be messy and expensive, if you could not isolate it.

For both cases, the valve is an important system component. 


  is this something better left to an expert?


It is a simple job, but depending on application and your locality (plumbing/boiler code) it may not be legal for someone other than a pro to do it. Check with the local Authority Having Jurisdiction (AHJ).