Most professional singers will use a vocal coach so it doesn't mean that you have a bad voice if you decide to use one! It just means that like any actor, artist, sportsperson or athlete you are looking for someone to help you, train you and work on those areas that you would like to make even better.
A vocal coach will look at and listen to your breathing technique - how are you breathing, how is the air getting into your body, where is it going and how are you controlling it? This will help with your volume, your tuning/pitch and your range.
They will also look at how your are using your vocal chords - are you tight and a bit 'shouty' or have you got a nice relaxed lower jaw so that you aren't straining when you sing? This will help give you a fuller sound and also help with those higher notes.
A vocal coach can also help with your tone by showing you how to put strength and depth into some notes and brightness into others.
They aren't there to 'change' how you sing, just to help you get the best voice you can - the aim is that you are then not limited by your voice but that your voice becomes something you can use in a variety of ways and with confidence.
If you have a part of your singing which you are a little frustrated with or you think there are areas you could improve then give some singing lessons a go and get yourself a vocal coach who can work with you.
More information on this site and also at http://www.singing-lessons-in-bristol.co.uk/