As I understand it, superconductors work best at temperatures approaching absolute zero (-273.15C). For example, Google tells me that the superconductor in an MRI operates at -269C.
There has been a lot a buzz lately about room temperature (25C) superconductors being discovered, but why is room temperature the focus? Why not focus on superconductors that work in reasonably cold environments? For example, we can easily get temperatures to -15C in a freezer. Why not create superconductors that work in that temperature range rather than 25C?
Short answer is power lines. Tons of power is lost as heat between the power plant and your home. If they can make a room temp superconductor then every powerline could be replaced with a line that has no losses. This could mean power plants can be smaller and still supply the current power requirements or they could serve more people with the same size power plant which would hopefully bring down electricity prices some since they have more customers.