Just like people, plants need proper nutrition not just to survive, but to grow big and strong. There are nutrients in both potting and ground soil but over time the plant absorbs all the nutrients that are there to be had. For this reason, it is imperative that you follow a regular feeding schedule for your plants, to replenish lost nutrients in the soil.
All plants are different, and trees and shrubs do not require the same nutrients as flowering plants do. If possible, it is best to pick a plant food specifically designed for certain types of plants, like shrubs, fruit trees, flowering bushes and so on. These are formulated to meet the needs of those plants. If this is not possible, any type of plant food is better than none.
Generally speaking, it is best to feed your plants twice a year in spring and fall. There are products that can be used during the summer months, but be careful, chemicals can sometimes burn plants when it is hot outside.
Another way to feed your plants without using chemicals at all is to use compost. In order to substitute as an effective fertilizer, the compost needs to be rich (not just leaves, but other organic matter as well). Apply this compost to your beds in the spring like you would new soil. The nutrients in the compost will leach into the soil and provide nutrition for your plants.