What is the purpose of Eliot's "Conclusion?" Why does the novel not simply end with the deaths of its two central characters?

The postscript "Conclusion" serves to place Maggie (and Tom) in the larger context of their landscape and society. The same flood that took their lives has left its mark on the countryside, implying that Maggie and Tom's deaths left a mark on the community. The "Conclusion" also assures us that Stephen and Lucy have begun a new relationship and that Philip remains faithful to Maggie's memory through a lifetime.