On a functional level it's the fastest & the most convenient input method when you're not in front of the proper keyboard. Which even for me constitutes a big chunk of my life 🙃.
Taking a voice note allows me to quickly close an open loop of a stray though or a new idea and go on with my life mentally unburdened
Another important aspect is that taking voice notes evokes a certain experience of "fluency" for me.
That is because when you're taking a voice note, you can record a stream of consciousness without having to re-formulate or filter things.
Which allows you to avoid switching away from the context of original thought/breaking the flow to enter an "editor mode".
This is the reason that I'd occasionally take voice notes even if I'm in front of the proper keyboard.
See Iterating fast: voice dictation as a form of babble for more on this.
It's worth noting that I'm specifically talking about taking a voice note vs doing voice typing/dictation in this context.
Voice notes allow me to better harness the fruits of diffuse thinking mode (shower thoughts)