5 Benefits of Belief in al-Qadar (Pre-destination)
1. Peace of mind.
Everyone knows that what hit him could not have missed him and what missed him could not have reached him.
2. It doesn't make you grieve about anything because you don't say things like "if I did such and such then such and such would have happened",
so don't worry too much about the future because you believe that everything is already written. What you do worry about is your deeds in doing good.
3. It gives you the will power and determination to go forward in the way of Allah.
You don't fear anybody because you know that what hit you could not have missed you. The Prophet (sallallahu alaihi wa sallam) said to ibn Abbas: "if the whole nation were to gather together to benefit you they would only benefit you with that which Allah had already written you and if the whole nation were to gather to harm you they could only harm you with that which Allah had already written to harm you. The pen has been lifted and the ink has dried."
4. It teaches you to be modest and humble because everything that you are doing is by the will of Allah
so if you succeed you know that Allah helped you in doing it, you don't succeed because you are intelligent or because you are rich or because you deserve it and a person isn't poor because he is stupid. This stops you from having pride or arrogance because you know that Allah can afflict you with harm.
5. Total dependence upon Allah.
You take precautions, do as much as you can and then depend upon Allah because al-qadar does not stop you from taking precautions.
Allah (subhanahu wa ta'ala) said: "Whatever of good reaches you is from Allah and whatever of evil befalls you is from yourself."
The Prophet (sallallahu alaihi wa sallam) said: "Take precautions as everybody is guided to that which has been destined for him. ", i.e. you should drink to quench your thirst If something bad comes it's because of our bad doings and if something good reaches us then it is from Allah grace.
Taken from: http://www.islamweb.net/