The "c" in E=mc2

The speed of light can't be exceeded without causing some kind of weirdness to the universe as we currently, physically understand it. However, it can be slowed down. But what good is that?

According to io9 there are two reasons. A1, fictionally:

Bob Shaw's 1968 short story "Light of Other Days" proposed an interesting use for materials with a high refractive index – use slow glass with a refractive index so high that light takes a year to pass through it to "record" pleasant nature scenes. Then people living in drab cities could place these "scenedows" in their houses and enjoy a year's worth of lakeside bird watching instead of the laundromat across the street. L. Sprague de Camp had weaponized slow glass technology earlier, in "The Exalted," printed in a 1940 issue of Astounding. He used a rod that trapped light, then released it all at once to explosive effect.

And A2, in practice:

Modern researchers have found an even more powerful use for slow light. In the last ten years, scientists have managed to slow light to a crawl and even stop it using special gas diffusions excited in particular ways with laser beams. When scientists stop light, they aren't actually halting a photon – they're embedding its quantum state into the nearby atoms. Later, they use another laser pulse to activate those atoms and make them emit an identical photon. And by later, I mean almost instantly, because the quantum state starts to decay in less than one second. The ability to slow, stop and generate photons in this way is a major step on the road to developing quantum computers.