While stuck on Earth for much of his Third incarnation, the Doctor thought of himself as some kind of galactic yo-yo as he would always return to the aid of the human race. It seems to still be true, even after the Time Lords lifted their control over his TARDIS and allowed him to roam free. Perhaps this is why the Doctor has favoured passing the time with a yo-yo so much. The Fourth Doctor was often seen practicing his tricks and bounces whenever he had a spare moment, but they're not without their practical uses, too. In The Ark in Space he used it to test the local gravity, which he did again in Kill the Moon. See. Yo-yos are fun and resourceful.