Leogtr, that's because you have live plants and things are balanced, relatively anyway.
Kishii, algae is a plant, and all plants need nutrients in the presence of light. Without live plants in your tank, there is nothing to use the nutrients that are there, so algae appears. The more light you have (in intensity and duration), the more algae.
Nutrients occur from organics--fish waste, uneaten food, decaying plant/animal matter, etc. Perfectly natural. There are bacteria that will appear automatically and these break down the organics--in the filter, in the substrate, even on any surface. The nutrients that result provide food for algae/plants.
Algae in the absence of live plants is actually a good thing, as it is one natural way to reduce excess nutrients which is better for the fish. Regular weekly partial water changes are another. Not feeding more than sufficient and not overstocking fish are other ways to limit nutrients and the resulting high nitrates.
There are many types of algae, some can be quite attractive, and natural looking. Those of us with planted tanks don't like seeing too much algae because when it starts growing on plant leaves it inhibits respiration and nutrient uptake and the plants can fail and die.
Limiting the light works to minimize algae.