I was watching one of the broadcasts of a space walk on the ISS. I noticed the Pistol Grip Tool (Thanks Courageous Potato) looks like its made of polished stainless steel. Why do these tools have shiny metalic surfaces? Is it for visibility or heat dissipation?
It's not "heat dissipation", but rather, the opposite: preventing excessive heat absorption or dissipation.
Shiny metal surfaces (or surfaces covered with shiny metal paint or tape, as its stated that the PGT itself is made of plastic) reflect most of the heat of the Sun when exposed to the sun, so they only slowly warm up. And when they are in the shade, they only slowly emit radiation and cool down. This is also true of white surfaces, which are simply diffusely reflective rather than specularly reflective.
This is also the reason why many spacecraft are covered with a crumpled shiny foil material -- multiple layers of shiny non-emitting, non-absorbing material form a strong heat insulator when in the vacuum of space.
(There is also a more prosaic reason for space equipment to be made of shiny metal. The vacuum environment tends to favor metallic and ceramic materials, and these types of high-quality custom equipment are often going to be made out of metal that is machined and then polished, nickel-plated, coated with clear anodize or iridite that leaves it shiny, etc.)