The angel here is an element of his later paintings of the Annunciation. He would depict this famous Christian scene on several occasions. Artist Titian was looking to perfect the anatomical elements of this leading figure through constant, targeted practice.
Titian's drawings could also sometimes be passed around his studio in order to help others learn his methods and also perhaps provide guidance when they are completing some of his commissioned pieces. In the modern day the art of drawing is far more closely studied and considered an art form in itself.
The common tools for drawing during the Renaissance were chalk and charcoal, with this piece likely to have been produced using one or the other of those. Titian tended to prefer darker lines, where as others, such as Da Vinci, would prefer red chalk instead. They would also experiment with different types of paper too.
Each of Titian's Annunciation paintings are particularly complex compositions and so it was essential that he practiced the challenging elements of them separately. An experienced artist is able to crop out parts and work on a certain pose or facial expression before implementing the preferred style into the final painting.