Why does case-pattern undergo "process substitution"?

[shynur .]

Hi, friends~

I'm reading the Bash Reference Manual, section 3.2.5.2.
Here: <https://gnu.org/s/bash/manual/html_node/Conditional-Constructs.html>.

    case _word_ in
        [ [(] _pattern_ [| _pattern_]…) _command-list_ ;;]…
    esac

    Each _pattern_ undergoes tilde expansion, parameter
    expansion, command substitution, arithmetic expansion,
    process substitution, and quote removal.
    ^^^^^^^^^^^^^^^^^^^^

I'm confused.  Why is "process substitution" being performed here?
The result is often unforeseen, e.g., '/dev/fd/13', '/dev/fd/42'...
Can anyone give me a practical example?  Thanks in advance!

--
shynur

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