Exhibition tennis events have become quite popular in recent years, which is why we're seeing more and more events being hosted. There is a simple reason for that: there are no rigid rules.
The matches can be any format, as seen with the ultra-popular UTS model. The players can play either singles or doubles and in any combination they want, which generally isn't the case in events, at least not for most of them.
Where else would you get spouses Elina Svitolina and Gael Monfils competing in a tennis match? You can only get that in Grand Slams, and there are only four of those.
It's also a nice way to experience a tennis match when everybody is a bit more relaxed than usual. There is joking, there is laughter, and it's just a really great time compared to the very serious nature of a proper tennis match.
Now, obviously, they're more than just spouses; both are pretty good tennis players in their own right, especially Svitolina, who is closer to her peak than Monfils, who is, in fairness, heading to retirement sooner rather than later.
Rune posted the promotional poster for the match on his social media, and for the Dane it may be also a preparation before teaming up with Wozniacki at the Paris Olympics.