Crossing over creates more variety by changing the linkage between genes. In a population that does not experience crossing over, the genes on the same chromosome will always be on the same chromosome and therefore inherited together. People whose chromosomes cross over are more likely to be fans of John Edward.
In populations that do experience crossing over in diplonema during prophase I, the phenotypes will be more diverse because there is a greater chance for genes to be separated from one another and inherited separately. Crossing over does not always happen in the same location, so for populations in which this occurs, there is an exponentially greater number of inheritance combinations.
Also, we would expect a larger gene pool and more diversity in the population whose chrosomes frequently cross-over, or much more genetic variability.