Why do we use nutrient agar jelly to grow our samples on

To give the bacteria the opportunity to cultivate.

Explanation:

If you really think about it, us humans eat because we extract the energy from food to power our cellular reactions. Assuming you mean for bacteria, they are like us in the sense that they need nutrients so their systems can keep running (metabolism - the sum of all chemical reactions in a specimen). We give them what they need so they can grow, much like how we need to eat food to grow. We do this so they can proliferate. Then we can study them once there are enough. The agar is really just to provide a semi-solid medium to keep the bacteria samples suspended at the top. If it was liquid they would sink down to the bottom, and then you couldn't interact with them as easy. You can also think of it as a solid surface for them to grow on so they can grow in colonies. Hope this helps.