I think one possible point of confusion is whether we are looking at an amp with a dedicated 70V output or a low impedance amp being used to try to direct drive a 70V line. Simple Ohm's Law that basically any amp rated 625W or higher into 8 Ohms (1227W into 4 Ohms) can be used to direct drive a 70V system but below that the amp needs an output transformer to step up the voltage.
I bring this up as the choice of amplifier may have more to do with wanting enough output for the connected speakers and transformers, plus some for line losses, while also being able to provide a 70V (actually 70.7V) output. Would a 350W or 400W amp be sufficient for 300W of load, probably, but you may not find that many cost effective 350W or 400W rated amps with 70V outputs. But a 625W amp could provide plenty of power for the load and direct drive it as a 70V system.
70V (and 100V, 140V, etc.) systems were developed greatly to reduce cable losses where you had very long runs, much like why power lines are stepped up to high voltage and then stepped back down at the endpoint. This could definitely be a factor a venues like a stadium, airport or raceway.