What are VDDA and VSSA?
VDDA and VSSA are the separate analog supply and analog ground pins for the MCU.
These pins are directly powering only the analog components such as ADC inside the MCU.
It differs from the digital power supply so that the digital supply pins power all the digital components in the MCU.
They are separated so the user can route the power pins separately and filter any digital noise so that the analog supply is clean and does not have much noise which could affect the ADC reading for example.
So it's not a better supply, the MCU needs all power supply and all ground pins to be connected. It just powers analog components from separate pins.
As the analog and digital parts must communicate between eachother inside the chip, the analog and digital supplies must be within range given in datasheet for the part to work. In this case, the VDD and AVDD must be connected to same voltage supply, as they are not allowed to differ more than 50mV when running, and up to 300mV during power-up and power-down transitions.