In the real world, people don't exist inside a bubble; there are events, actions, and storylines happening around them. In a book, it should be the same scenario. Whatever is happening to the main character, whatever journey he/she is embarking upon, should be complimented or challenged by external forces. Very often, this will mean a subplot.
I think Tracy said it well yesterday when she commented that subplots and secondary characters--a book's supporting cast, if you will--often go hand-in-hand. For me, it's because the secondary characters are often the ones who are acting out and involved in those subplots.
In a Bump in the Road, Clare's best friend Reese is going through a difficult marriage and a surprise pregnancy. This allows Clare to realize that life has many curveballs, and while she herself is unexpectedly pregnant, her marriage is stable and happy. Seeing her friend's challenges reminds her to be grateful for her blessings.
On the other hand, Clare's friend Julie is a party girl and still going out and having fun every night of the week. She meets a new man throughout the book and endures the challenges of a new relationship. Hearing about Julie's late-night escapades and adventures, reminds Clare that she's entered a totally new stage in her life, and will have to grow into her new role as a mother.
What worked about these subplots is that they both stood on their own. As in, they had their own narrative. However, they directly connected back to the main storyline by either mirroring the main character's emotional journey, or conflicting with it. That's truly the key in an effective subplot: it needs to be relevant to the story, either emotionally or externally.