Speaker systems: When is 25 volt used vs 70 volt?
In a nutshell - cost.  25V distributed speaker systems most often use the existing 16 Ohm transformer winding found in older PA amplifiers. 

In addition, building the matching transformer that mounts on the rear of every speaker in the system costs less if it has to handle only a 25V input rather than beings required to handle the 70V or 100V systems most often used today.  Less wire / turns of wire equals lower cost. 

One final thought - a 25V distributed speaker system usually requires only Class 2 wiring. 

Note that many 70V systems can also use Class 2 wiring, so long as the total power capability of the amplifier is 100 Watts or less.