Knowing your purpose gives meaning to your life. This is why people try New Age methods like astrology and psychics to discover what life is all about.
But when your life has a steadfast purpose you can carry on through almost anything. Without it, nothing is bearable.
Isaiah complained in Isa 49.4 "I have laboured to no purpose; I have spent my strength in vain and for nothing"
Job lamented in the Good News Bible Job 7.16, "I give up; I am tired of living. Leave me alone. My life makes no sense".
Medical doctor Dr. Bernie Siegel found that he could predict which of his cancer patients were most likely to survive by asking, "Do you want to live to be 100?"
Those with a deep sense of purpose, and answered "yes", were the ones who would live.
Having a steadfast purpose makes life simpler. It defines what you do and what you don't do. It becomes the standard you use to evaluate which activities are essential and which are not. Without purpose you have no real foundation on which to base your decisions, allocate your time and use your resources. You tend to make choices based on circumstances, pressures, and whatever your mood is at that moment.
When you don't know your purpose you try to do too much, and that causes stress and conflict. It's impossible to do everything people want you to do; you have just enough time to do God's will. If you can't get it all done, you're trying to do more than God intended.
A steadfast purpose makes your lifestyle simpler and your schedule saner.
And it leads to peace of mind: Isa 26.3 "You will keep in perfect peace him whose mind is stayed on Thee".
Stayed means steadfast.