This is something I learned about too. Apparently, even though rims are round, they are not perfectly round and not perfectly weight-balanced. As such, they do this thing called balancing, where they try to make everything perfectly weight-balanced by putting those 'chicklets' where weight is needed. Otherwise, if not done, when your rims are spinning at speed, there will probably be some vibrating.
EDIT: this thread kinda reminds me of a similar one from
here. "...
circle metal thing..." No offence intended.
