In dhyaan/ mindfulness meditation, you are usually asked to focus on physical sensations in your body (breath, sounds, surfaces in contact, etc) to control random thoughts of your monkey mind.

This has 2 advantages:

1) Focusing as well as maintaining that focus on a real, physical sensation is much easier compared to an abstract idea or imaginary picture/ person.

2) If you're imagining a leaf, next moment you may imagine a goat coming...