Answer
This reproductive strategy does two things: 1) it promotes outcrossing, with the genetic benefits of hybridization, 2) it provides a backup in case non-self pollen is unavailable, allowing at least some reproduction to occur.
Work Step by Step
Note that the book extolls the value of self-incompatibility, which this system resembles in some ways. However, it also makes sure that the plant will produce some seed/reproduce itself, even if pollination from other flowers is poor.