I think this might a combination of early camp experimentation and injury necessity. This offense isn't going to feature a FB the way a McDaniels offense did, so they're not (likely) going to carry a true FB on the roster. But there will be times on the goal-line and 3rd/4th and 1 where you're going to want someone in that lead blocking role.
Typically, you'd just throw an extra OL in that spot, but injuries have them kind of thin there right now. You could also put a TE in that role, but in those specific situations you're typically also using three TE's in the formation. So unless you're planning on carrying 4 TE's on gameday, practicing that isn't going to really help much. So they're trying out Tavai to see if it can work.
We've seen them do similar things in the past, either for similar reasons or because their actual FB was injured, like when Elandon Roberts was filling the role.