What's the idea of specifying binary=true together with textconv?

Hello,

When textconv is specified, the program is always used for conversion.

E.g. consider this:
[diff "png"]
  textconv = wc

Even if a png file is ASCII (manually created), wc gets called, and
the result is made of its output.

So, the question is: why should one specify binary = true together
with textconv?

There is an example in docs:
[diff "ps"]
  textconv = ps2ascii
  binary = true

...But what binary = true does here?
Even if we remove it, textconv is called for all ps-files.

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Best Regards,
Ilya Kantor