Most teams don't redshirt a player who is a year from starting. So if you have a senior center and a true freshman you might play the freshman. The following year because of min requirements you will have a true frosh or walkon and I try to avoid having two players at the same position same year (wr, ot , og and cb) are my exceptions to that.
A highly rated recruit may play as a true freshman knowing if you redshirt them as a freshman they may leave as a RS sophomore or RS junior so you might as well get as many years of them as you can.