Some amps use a technique called 'power scaling' on the output power tubes to allow lower-volume playing with full-power sound. It reduces all of the voltages (except the heater filament) by the same proportions, so (in theory at least) it should distort in the same way, while making less power. This technique will actually extend tube life, because they are dissipating less power when they work at lower voltages.

Attenuators connect between the amp output and the speakers. You can crank the amp full-pin into the device's input, but only a small portion of that power is passed along to the speakers. Blues players lose a lot of the 'touch' with this rig, but it keeps the neighbors peacefully unaware. Placing an attenuator between the amp and the speakers will reduce some of the 'springy' response that lets a bluesman ride the edge of power-tube distortion. If your tone is mostly derived from pedals or pre-amp distortion, an attenuator is a wonderful thing. I recommend the Weber MASS as the best of the bunch. They get high marks from even some of the most notorious cork-sniffin' tone purists.

One caveat: Because an attenuator allows you to reduce the sound level while pushing the amp hard, you will probably push the amp hard more often. This will definitely impose a harder life on your tubes, and they will die more quickly. The attenuator doesn't harm your tubes. Playing with the amp cranked all the time is what does it in.