Usable websites require less psychic energy to be used

Users will typically be coming into your websites to accomplish a certain task. The easier it is for your websites to be used, the less Psychic energy is going to be used by the users. When the more psychic energy is needed, users will get frustrated and leave.

This is why Web usability is achieved by designing for the intended attention types. Adding long instructions, for example, will drain the users energy unnecessarily (Self-evident websites do not need instructions).