What is this purple plastic ring on the center bore hole on my wheel?

https://imgur.com/a/gsP6L4f I assume it has something to do with correcting a mismatch between the wheel centerbore and the hub diameters. Is having it structurally important for the vehicle? If I lose one, how important is it to have and where can I find replacements? How are they sized and how do I measure their size?