I love to paint mechanical things- even though I may not always be successful, it is the most fun. The bicycle required some close examination in order to draw/paint it so it appeared to actually work, as a bike.
The simple stack of books and goblet involved some hard LOOKING and an understanding of perspective in order to draw it correctly. Accuracy in the drawing is foundational to better painting; and anything that makes you look, and look and look again - will stretch and improve your drawing skills
- but then so will using a sketch book.