No one is required to use it and even when you receive a mail tagged VSRE, you may still decide to write a long reply if you can not communicate your messag in a few words.
The obvious use case is you are communicating with someone for the first time or for not so long, you are fine with a short reply and you know that recipient would like to write a short reply but will not do so in order not to sound impolite.
The obvious use case is you are communicating with someone for the first time or for not so long, you are fine with a short reply and you know that recipient would like to write a short reply but will not do so in order not to sound impolite.