why --output=dir option forces change to dir before running lilypond?

[Federico Bruni]

Hi all

I've already started a discussion about --output on -user:
http://lists.gnu.org/archive/html/lilypond-user/2018-03/msg00301.html

There is more than one issue in that thread.
Anyway, what I'd like to know is why --output force the change to 
output directory before running lilypond.
I'm playing with lilypond and automation tools (such as Make and 
recently Meson) and it seems that this peculiarity of lilypond is 
breaking my simple projects (which work well in similar situations, 
where the "compiler" from text to pdf -- pandoc -- does not behave like 
lilypond does).

There's any good reason for this behaviour?

Thanks in advance
Federico