As Mufti Abdur Rahman ibn Yusuf said in the talk on Depression, dua wards off calamities.
A bit like this.
Look at your Dua's going up and on its way are calamities coming to you. Your Dua's intercept these calamities and protect you!
MuftiARM said if you don't believe that, try not to make dua for a week and see what happens.
AND I'M NOT SUGGESTING YOU TRY THAT :)
Dua is most definitely a weapon for the believer!

