Vdd and Vss pins microcontroller
Good grounding helps a lot when reducing electrical noise in circuits and it also provides additional return paths for the current that your microcontroller pushes out while driving things (LEDs, transistors, signals to other parts, etc.) These extra ground pins help with both of these things. Sometimes, chips like these use multiple external pins to ground different parts of the die inside the part (so they don't need to use valuable silicon wafer area just for routing ground everywhere inside the die).

Additionally, if your micro only had one pin on it for ground then all of the current it pushes out would need to return on that one pin and may overload that pin. The reasoning for multiple Vdd pins is similar.