Plants that grow out in the yard will get all the nutrients they require from the soil and the dead leaves and fading flower heads will create mulch that will rot down adding even more goodness.
For containerized specimens it is a very different matter, as the soil will lose its minerals over a season whether out on the porch or indoors. [Container gardening]
Thankfully, there is a huge range of plant supplements often specifically designed for different plant types. It is often known as plant food and contains various substances recognized to be good for plants.
There are three main elements that a branded feed will be made up of and often the one listed is nitrogen. It promotes good healthy plant growth and is easily dissolved in water, however; it can also be hard for it to be absorbed, and therefore many popular types will have other chemicals that will deal with this problem.
Next in line is phosphorous which is not actually what it seems it is a scientifically a percentage of phosphoric acid. It is more easily absorbed by plants with less leakage than nitrogen.
Lastly, on the ingredient list will usually be potassium, the combination of oxygen and potassium makes a substance called potash. This encourages growth in particular the amount of flowers that a plant will produce. Healthy plant with vigorous growth will be the reward of extra potassium
If you have plants that have been bred to withstand certain disease and resist cold conditions then potassium at the right levels will help them along.