The character without a purpose

This is a big no no as I have read.  Although off the page, mysteries abound, none can be left in the story.  Everything has its reason and purpose.

I wanted a character to wander as we do in real life. We  all stroll, get lost and ,hopefully ,don´t end up in a ditch.  The writing gurus shake their heads in unison. ´Motive´  they chant.  You would think finding a motive for every character would be simple. There would be those characters who want to be loved or avenged etc, but what happens to those who don´t know what they want?

This is where the gurus and I differ. Our real world is filled with those who are half sure or not completely sold on the idea or situation. The question is -  how do you factor them into writing?