Are they absolutely necessary--no, but they are the best way to fertilize container tomatoes--or just about any container plant. Water soluble fertilizers (MG, 20-20-20, fish&seaweed) will work too, but can lead to a roller coaster effect of excess nutrients followed by a deficiency if relied on as the sole source of nutrients. My general rule is to use the slow release fertilizer as the foundation of plant nutrients in the container, then add a soluble fertilizer as needed during peak growth periods. Soluble fertilizer will especially help to add micronutrients, since most of the slow release pellets don't contain micros.
Whatever you do, don't count on the container soil itself for much in the way of plant nutrition. I don't use any manure or compost in the container soil I mix myself due to the fact they can impede drainage and aeration.
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