Smaller distance to travel so signals get from one gate to another quicker which will enable a higher clock speed. Smaller devices use less power so less heat

Smaller distance to travel so signals get from one gate to another quicker which will enable a higher clock speed. Smaller devices use less power so less heat and longer battery life. Smaller devices mean a smaller chip which is cheaper (although mask costs will be more expensive) or use the extra area for more features like more cache or another processor core.