It is supposed to block the blows that go between the plates, not reinforce the plate itself. A lighter alternative that developed later is patches of chainmail only between the plates.
Mail armour is a layer of protective clothing worn most commonly from the 9th to the 13th century, though it would continue to be worn under plate armour until the 15th century.
In the chapter "Late Middle Ages":
In armoured techniques taught in the German school of swordsmanship, the attacker concentrates on these "weak spots", resulting in a fighting style very different from unarmoured sword-fighting. Because of this weakness, most warriors wore a mail shirt (haubergeon or hauberk) beneath their plate armour (or coat-of-plates). Later, full mail shirts were replaced with mail patches, called gussets, which were sewn onto a gambeson or arming jacket.
https://en.wikipedia.org/wiki/Hauberk
Translated from german with DeepL:
https://de.wikipedia.org/wiki/R%C3%BCstung_(Schutzkleidung)
Here is an example picture of a Milanese Armour: https://en.m.wikipedia.org/wiki/Giano_II_di_Campofregoso#/media/File%3AHJRK_A_11_-_Armour_of_Giano_II_di_Campofregoso.jpg