People use dropdowns to improve their smoking experience for a variety of reasons.
Some people use a dropdown because the bowl of their piece is normally too close to their face, and they burn their hair or eyebrows. To prevent this, a dropdown increases the distance between their face and the piece.
Others use a dropdown because their bowl or banger is too close to another part of their piece, and they want to increase the distance to avoid damaging the pipe itself.
Whatever your reason, dropdowns are a great way to help protect yourself, your piece, and to keep your pipe cleaner.