Adequate nutrition and hydration during activity provides the energy needed to perform at a high level, allows for mental focus and skill execution, and helps prevent hunger and thirst. The duration and intensity of your activity will affect your food and fluid needs, as well as the temperature, humidity, and altitude at which you are being active. This is a very individualized science and your needs may differ from your workout partner or teammate.
It's important to start to make healthy food choices on a regular basis, but once you add more activity to your life it's important to look at your food choices around your activity. With more activity comes a higher sweat and energy loss. These fluid and food losses need to be replaced to fuel your next workout session.