Plants in pots have a finite amount of soil to live in. They change the soil. They remove minerals, trace elements, and nutrients such as nitrogen, phosphorous, and potassium. Also, watering a pot, the water picks up water-soluble things (fertilizer, minerals, etc) and over time flushes these out the bottom of the pot. Furthermore, soil in a pot breaks down into a finer and finer texture, which changes its structure making it more dense. This is why you add compost to the soil in buckets every year. (Soil out in the world gets that sort of addition (leaves, dead plant matter, etc) naturally.) Compost adds new nutrients and better structure to the soil. Take some out to make room for the new compost.
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