The LCP is designed to be a lightweight pocket pistol for self-defense use and most defensive encounter are solved with only a few rounds. So while some people choose to carry only one magazine the prepared individual will generally carry at least one backup magazine. With the diminutive size of an LCP magazine carrying a second is easy so it is a good idea to have at least one extra just in case Murphy rears his ugly head and tries throwing a wrench in your works.
Extra magazines come in handy at the range and extra capacity magazines can reduce the number of mag changes and allow you to increase the length of your firing strings making practice easier and much more enjoyable.