Commercial 70 volt amplifiers are designed to be used under the most demanding circumstances. Some commercial pro audio amplifiers only drive 70 volt (distributed line) systems, while some can also drive low inpedance systems ( 8 or 4 ohm) speakers. The main advantage to a 70 volt speaker system is that you can run a tremendous number of speakers off of a single amplifier.
For instance, if you have a commercial 70 volt amplifer that can generate 1,000 watts of power, you could run 100 speakers that each required 10 watts. This is not a perfect example as you should leave some headroom, so you would probably want to use that 1,000 watt amplifier to run 100 speakers tapped at 8 watts a piece, leaving you a decent amount of headroom to avoid distortion.
70 volt systems are used in warehouses, office buildings, churches, and grocery stores. Basically anywhere you need to run a large amount of speakers with as few amplifiers as possible.