Final answer:
NAND and NOR gates are considered universal gates because they can be used to create any other type of gate, providing functional completeness for digital circuit design.
Step-by-step explanation:
The reason why NAND and NOR gates are the most used logic gates and categorized as universal gates is because they can be used to create any other type of gate.
This means that any digital logic function can be constructed using just NAND or just NOR gates. This is a property known as functional completeness. In digital circuit design, versatility and reduction in component count are valuable. By using only one type of gate, it is easier to design and manufacture the circuit compared to using multiple types of gates.
Additionally, while simplicity, speed, and power efficiency are important considerations in logic gate design, the universality of NAND and NOR gates is primarily due to their ability to replicate the functions of all other logic gates, such as AND, OR, NOT, XOR, and XNOR, through specific combinations and interconnections of these gates.