Federer playing on clay
https://www.telegraph.co.uk/tennis/2019/02/20/roger-federer-play-madrid-open-clay-comeback-confirmed/
The past couple of years Federer skipped the clay court season, due to his age, the beating clay takes on a body, and wanting to be fresh for the grass court season.
I think this is his farewell tour, he is gearing up for the French Open, which will be his last. I can't think of any other reason why he would prepare and play in the FO.