Purpose of capacitor for MCU reset pin
It is for "debouncing" of the pushbutton.  Virtually all pushbuttons, when pressed, do not make a single clean contact, but instead make several repeated contacts within a period of 10-50 ms.  This would cause the microcontroller to begin to reset several times on each pushbutton press.  The capacitor suppresses this.

For more information on debouncing, see this article.