Stability is built in diversity. This is true for all systems- human or otherwise.
Monoculture farms require such large quantities of pesticide and herbicide because there is no diversity. Adding flowers would attract beneficial insects which would consume populations of pest bugs. Increasing plant diversity would improve the soil and foster healthy microorganisms that would prevent fungal pandemics.
In communities of people, diversity of cultural heritages allows the community to be more creative and resilient to change. If all of the people in a community think the same way, then they will all come up with the same solutions to the same problems. By having diversity in community, we can create better solutions to problems and be more adaptive in the face of rapid change.