The more "higher level" thought processes required just to operate the basics of your device (e.g., basic navigation, app launching, scrolling, etc) the more mental effort (and thus, frustration and cognitive load) it takes from the user.

As an extension to this, the less brain power you have to devote to basic, fundamental interactions with the machine, the more bandwidth you have to dedicate towards other things (e.g., if the calculator is hard to use, you're spending more effort wrangling the calculator instead of solving math problems).