Commercial fertilizers are already tested, so you know it's going to have this much nitrogen, phosphorus, and potassium. With this, you can pick a fertilizer based on the needs of your plants. When you're making fertilizer at home, it's hard to know exactly what the chemical makeup is; plus, you can end up burning the plant if the homemade fertilizer has too much nitrogen or if the compost hasn't cooked all the way or cooled down.