Why do universitys do this?

They give you an offer, so you still have to get the grades unless you get an unconditional.

So even if you dont get an offer in march they can still accept you later?

Oh right so they will give someone an offer in march but if they dont get the grades for example it is withdrawn. The person who gets an offer later will only get it through clearing? Sorry if I sound new I literally never knew about this

If someone does not achieve the required grades in their conditional offer, then the Uni can use their discretion and accept them anyway. There is no guarantee that this will be possible, as it depends if they have places left after confirming spots for folks that met their offer.