You dont even need them. You just think they make you look a bit clever. Several Doctors have donned the spectacles to give themselves that extra air of authority. The First Doctor, back in The Daleks, had a pair which he used from time to time but he generally preferred his monocle. Having recently regenerated, the Second Doctor briefly carries them in The Power of the Daleks but never puts them on. In fact, fans dont see the spectacles again until the Fifth Doctor uses a Dumbledore-style half-moon pair in Castrovalva, which are presumably the same glasses which later show up in the Seventh Doctors Silver Nemesis adventure. But it is the Tenth Doctor who gave the glasses plenty of screen time, as well as their signature name of brainy specs (Time Crash). First wearing them in Tooth and Claw, the glasses quickly became a staple of the Tenth Doctors attire. Go to any convention and at least half of the fans dressed as Ten will be wearing them.