why the c99 mandate?

[Mayuresh Kathe]

i am not a c hotshot, so pardon my ignorance.
i read that all new code under openbsd has to be c99.
may i know what's so special about c99 over c89 which has been under heavy
use for so long?
also, does the mandate hold true for modifications to old (c89) code?
thank you.