Power Rail swings on a Digital MCU
This is not about calculation as it is common off-the-shelf standards. For CPU / MCU voltage 3.3 volts is very common and at the upper end of what the TI CC2640R2F is rated for. Yes it can work at 1.8 volts but with limited maximum speed, while 3.3 volts should allow for maximum clock speed.
Capacitors are inserted because ALL electronic devices can be noisy, with the outputs causing current spikes at the power pins. Normally each power/ground pair has a local 100 nF capacitor and close by is a 4.7 uF capacitor. Common for the board would be a large 100 uF capacitor to act as a current reservoir.
If a local voltage regulator is used it also must have bypass capacitors on the input and output pins. Normally manufactures show some simple build examples for CPU's/MCU's and regulators to show power/ground layout and capacitor locations.
A well regulated supply with adequate bypass capacitors should not even have noticeable noise much less voltage swings over 50 mV from the stated voltage of the regulator.