Compressor is most useful for people with weak voices - peoples with strong voices better to learn to stand back a pace if singing loud, near when quiet. Most people throw head back if singing loud anyway -gives natural microphone technique. Compressor useful if anyone singing quietly - not affected by size of venue. Size of venue only relevant to overall power of PA needed. Try renting compressor with invited audience and see what they think before buying. Two channels because sometimes useful for other purposes, or two backing singers usually. Good for improve dynamic control. Not make bad singer sound good. Make mistakes more obvious usually.