Hi Ed,
Compressors are prone to love/hate relationships among users. The design of a compressor is to "level" out all the outer edges of the dynamic range (high volumes and low volumes)and "squash" them into a more constant level, somewhere in the middle.

This allows for very little volume change with dynamics, but takes away some potentially dangerous loud peaks. It's a trade off, for sure.

In practical application - it's nice to have most of the material played at or near the same volume level, but that gets boring after a while. The most human element we can add to a performance is dynamic changes - take THAT away and it sounds canned or programmed. No life, if it's all the same level.

Much of the music in the top 40 radio world is highly compressed so that EVERYthing is loud and steady. Great for dancing, or to cover up the lack of talent in a weak performer. Ever hear a car comercial that WASN'T extra loud ?

As a solo perfromer, compression can be used to enhance the vocals if used in moderation. Try the factory settings first, and sing different volume levels - see which sounds the most natural. It's a very sensitive subject, and too much compression will make a signal sound just terrible.

I never compress in a live show, but in my studio I use it at a very slight level to allow more signal to enter the recorder. This improves the quality of the overall recording, and makes mixing much easier if no signals are bouncing into the red zone.

If your vocals tend to blare out in sudden volume changes - a compressor will help smooth out the level. If you are a more controlled singer, and can achieve the desired effect by mic technique - maybe you will not need to use it.