Fertilizing is not necessary, but it is a good option for succulent growers who want to encourage faster, larger growth. It can be used on mature plants with established roots. Only fertilize in the growing season (spring and summer for most varieties) to avoid weakening or burning dormant plants. Use a light touch and make 2-3 applications of low-Nitrogen, diluted fertilizer each year (N-P-K ratio around 5-10-10). You can find specially-formulated cactus / succulent fertilizers in most garden centers or go organic with compost tea or bone meal.
Example: Miracle-Gro Quick Start, diluted to half strength