What does the quote nothing contributes so much to tranquilize the mind as a steady purpose mean?

People can tend to panic, get confused, or just run around in circles if they're not really sure what they should be doing. However, if you have a clear goal in mind, it's much easier to focus specifically on that goal and the things you need to do to get there, and ignore irrelevant distractions.

Have you ever sat down to write a paper with no idea what to write and just stared at the blank sheet for a while, then gotten up to make a snack, watched some TV, sat down and looked at the paper again, given up and gone outside, etc? But, once you've decided what your topic will be and a basic outline of what you want to say- a "steady purpose"- it's much easier to buckle down and focus on getting the writing done.