Its because all twas and TCAS installations require a rad Alt to be fitted.
Most of them have DH callouts fitted to them. Most CAT I operators seem have it in there SOP's that you set it. Yes I know that CAT I is baro and if its not a CAT II runway its pointless. Been there and had that argument before.
The theory is that its a backup in case you have mucked up setting your QNH.
But if your going to bust mins why bother setting it at all.