This allows 32-bit packages to be found when the user has specified PKG_CONFIG_PATH for some other reason.
This also mirrors the way e.g. i686-linux-gnu-pkg-config is implemented on Debian, and possibly other distributions as well.
This also prevents 64-bit .pc files from being found. This was originally intended as a benefit [1], but can contribute to misdetection of headers which are not actually multiarch (e.g. GStreamer, although at the time that [1] was written that was a preëxisting problem). In general a distribution which provides .pc files for one architecture should be expected to provide them for any architecture that it actually provides libraries for; even if that was not true of Debian in 2017, it is now. I moreover assert it is better to fail to find a present library than to incorrectly find the wrong one.
Note that we can't easily use i686-linux-gnu-pkg-config, as would otherwise be preferable, for reasons also described in [1].
[1] https://www.winehq.org/pipermail/wine-devel/2017-June/118002.html