Berns (2001) shows that the human brain not only looks for the unexpected it actually craves it.
Activity in the nucleus accumbens, the part of the brain that is active when people experience pleasurable events lights up when there is a surprise.
Not all surprises are equal, they get different neurons activated.
Things that are new and novel capture attention.
Providing something unexpected not only gets attention, but also can be actually pleasurable.
Although a certain amount of consistency (at a website, for example) is a good thing, if people are trying to complete a task, providing novel and unexpected content and interactions is good if you want people to try something new, or if you want them to come back to see what's new.