Not only more play, but more satisfying play.
It seems to me most experienced couples are in this to assuage urges that can't or won't be scratched by play within their marriage. Sure, you can role play and talk about fantasies or past experiences all you want, and that will lead to great sex, but if you want to see what your wife would do with a big dick, even a large dildo won't give her the whole experience. (That's just one example, I'll leave others to the student.)
We all have different desires, and those desires can change from year to year, even day to day. Flexible non-monogamy allows us to satisfy those desires, sometimes.