DSDR

Because that makes more sense than mentioning air speed and altitude.

Is there really a question here?

Because :-

1. Voltage too low - customers lamps go dim etc.
2. Voltage too high - customers lamps go very bright then fail etc.
3. Frequency too low (voltage normal, or worse, high) - customers' transformers overheat. Most motors run slower in proportion to frequency.
4. Frequency too high (voltage normal, or worse, low) - Motor driven loads like fans and pumps run faster (% increase in load is twice increase in frequency e.g 5% frequency increase causes 10% load increase) and can be overloaded.
5. Everything "AC mains" electrical you buy is designed to work at nameplate voltages and frequencies with small variation from them. If distribution voltage/frequency were not coordinated with that, many would not work well or fail quickly.
   

Adjusted to be relevant to "power distribution system", meaning the ENTIRE transmission system used by utilities.

1. Voltage too low - Nothing works, everything eventually burns up
2. Voltage too high - Everything explodes, then nothing works
3. Frequency too low (voltage normal, or worse, high) - ALL transformers overheat. AC motors run slower in proportion to frequency, DC power supplies may not care.
4. Frequency too high (voltage normal, or worse, low) - Motor driven loads like fans and pumps run faster (% increase in load is twice increase in frequency e.g 5% frequency increase causes 10% load increase) and can be overloaded.
5. Everything "AC mains" electrical you buy is designed to work at nameplate voltages and frequencies with small variation from them. If distribution voltage/frequency were not coordinated with that, many would not work well or fail quickly.

Entirely true. Do not search for DSDR to find where the question came from...