Many possible reasons, such as the off chance Trump gets blocked from running. I realise its unlikely he will get blocked but there is still a chance it could happen and would have looked slightly more likely at the start.

Also to force concessions from Trump. Not because she would have posed an actual risk to him in the primaries but because he had to spend money defending his position. If there is one thing Trump is always short of, its money.

She also acts as a blocker to other candidates, who may leverage a stronger position if she wasn’t funded properly into lucrative positions in a Trump dictatorship in exchange for standing down.