You keep your chin down so it is not an easy target for your opponent. It's a hotspot for knockouts, so keeping it down helps in preventing them.
Also, The most important part is that you are putting it down, and not out and down. Your chin should not be touching your chest, unless you have one big ass chin. Furthermore, if you are uncomfortable or you cannot see your opponent, you have your chin down much too far. Find a position that it is not perpendicular with you neck, but also make sure that it is comfortable for you and you can see your opponent clearly without straining.
If you need some more help, try asking your trainer the correct position. If you are afraid to do so due to fear of ridicule, find another trainer. If you don't have a trainer, watch some pro matches. Hopkins does a good job of keeping his chin in order, if my memory serves me right.