I’ve consulted for a company in the past that had a continuous integration server set up correctly and building their project, but they made the mistake of having their developers decide when to start the build process. This misses the whole point. It’s called “continuous” integration, not “developer-decides-when” integration. Your continuous integration server should build your project and run its tests whenever a change has been made and as soon as possible. The benefit to this is faster feedback.
Even if you’re always on the ball and click that “build” button as soon as possible, is everyone else on your team this good? Will you remember to click “build” 100% of the time? Why would you even want to have to remember? Every manual step in a process is an opportunity for human error. Automate the process and worry about more important things.