Pure speculation but I think it served a purpose. They had changed their rule regarding two title fights on a single card. It had been that the heavier weight class got the main event slot while the smaller weight class got co-main. For McGregor they switched this and gave him the main event and Lawler/Macdonald Co-main.There also wasn't any good reason for that to be an interim title fight, but they wanted to hand out a belt come hell or high water, lol.
If they would have changed it to a non-title fight it would have looked bad having Conor in a main-event non title fight while Lawler/Macdonald fought in Co-Main. So I think they said "Fuck it. Just make it an interim shot". That way the card maintains its integrity (that is, two advertised title shots) and didn't have a weird lineup