Plots are quite pleasant for novel readers and even more pleasant for novelists, providing a structure for the writing and all that jazz. But subplots are the sugar and spice. As a writer, I don’t enjoy the main plot so much–once I’ve constructed the gist of the book, it’s difficult to change, and that element of the creative work is done. For me, subplots allow play and fluid creativity on the sideline of a novel.
Subplots bear especial importance in science fiction writing. My novels are set in the near future; their plots involve characters who live and breathe and grow through that future. They don’t necessarily think about how their own time came to be. But for me (and I expect for many other readers) there must be a link between our present and that future-present, and I tuck those links into subplots: an older person who watched those changes unfold, perhaps, or a holdover community that progressed in a different direction, or other minor characters whose wanderings explore a different part of the world.
In my reading and writing both, I notice that subplots serve a very important commercial-literary purpose: sequels! If a book does not wrap up its main plot, I am annoyed and less likely to feel the book was a satisfactory literary experience. But subplots can be left floating, a stem into a future full plot with its own sundry subplots and schema. My NaNoWriMo novel of 2010 ended up having a character work on the border crossing between Kansas and Missouri; her relationships there were minor details in context but contributed a lot to the sequel in the end of Novel2011. And someday one of the subplots from the border-crossing will spout into its very own main plot, making the annual a perennial.