"He cannot be questioned for His acts, but they will be questioned (for theirs)".
However, some of the reasons may be as follows. And Allaah knows best.
1) Through calamities, Allah tests the Believers:
a) Do people think that they will be left alone because they say: We believe, - and will not be tested. And We indeed tested those who were before them. [al-Ankaboot:2]
b) Or think you that you will enter Paradise without such (trials) as came to those who passed away before you? They were afflicted with severe poverty and ailments and were so shaken that even the Messenger and those who believed along with him said, When (will come) the Help of Allah? Yes! Certainly, the Help of Allah is near! [al-Baqarah:214]
2) Through calamities, Allah forgives sins and raises our status:
The Prophet (pbuh - peace be upon him) said: There is nothing that befalls a believer, not even a thorn that pricks him, but Allah will record one good deed for him and will remove one bad deed from him. (Muslim).
Also, he (pbuh) said: Trials will continue to befall the believing man and woman, with regard to themselves, their children and their wealth, until they meet Allah with no sin on them. (Tirmidhi- Saheeh)
3) Through calamities, Allah distinguishes between people:
By testing us, Allah filters the pure from the evil, the good from the bad, the true from the false, the believer from the hypocrite.
a) Allah will not leave the believers in the state in which you are now, until He distinguishes the wicked from the good [Aal Imraan:179]
b) And Allaah will certainly make known (the truth of) those who are true, and will certainly make known (the falsehood of) those who are liars, (although Allaah knows all that before putting them to test) [al- Ankaboot:3]
4) Through disasters, Allah warns us & reminds us so we may return to Him:
This is one of the greatest reasons behind calamities and trials, that Allah warns us to correct our mistakes and mend our ways. If we do so, it is good only for us. If we dont pay heed, surely, only WE are the losers.
a) Verily, We sent (Messengers) to many nations before you (O Muhammad). And We seized them with extreme poverty and loss in health (with calamities) so that they might humble themselves (believe with humility).[Anaam :42]
b) See they not that they are put in trial once or twice every year (with different kinds of calamities, disease, famine)? Yet, they turn not in repentance, nor do they learn a lesson (from it) [al-Tawbah:126]
5) Calamities and disasters strike because of our sins:
And whatever of misfortune befalls you, it is because of what your hands have earned. And He pardons much [al-Shoora:30]
6) Disasters and calamities are a means of punishment:
And indeed, We destroyed generations before you when they did wrong, while their Messengers came to them with clear proofs, but they were not such as to believe! Thus do We requite the people who are Mujrimoon (criminals) [Yoonus:13]
7) If one is patient, they are also a means of great reward in the Hereafter:
The Prophet (pbuh) said: On the Day of Resurrection, when people who had suffered affliction are given their reward, those who were healthy will wish their skins had been cut to pieces with scissors when they were in the world (when they see the great REWARD for suffering) (Tirmidhi- Saheeh)
8) Disasters and calamities are reminders that the world is only temporary:
If the world were free of calamities, man would love it more and feel content with it, and would forget about the Hereafter. But calamities wake him up from his negligence and make him strive for the place (al-Jannah) in which there are NO calamities or trials.
9) Calamities and trials remind us that we are SO weak:
Problems and disasters are a sign of man's weakness and his need for his Lord. Man cannot succeed unless he realizes his need for his Lord and starts beseeching Him.
10) Worship during hard times has a special favor and a special reward.
The Prophet (pbuh) said: Worshipping at times of tribulation and confusion is like migrating to join me. (Muslim)
11) Calamities make a person appreciate the blessings he has from Allah:
Blessings that come after pain, hardship and calamity are more precious to people. So then they appreciate the blessings as they should be appreciated, and thank Allah.
If things are always easy, a person may forget all the blessing given to him By Allah and not be grateful for it, so Allaah tests him by taking some of them away , as a reminder to him to be grateful for it. And, only the one whose heart is open will feel this. Those who have no heart do not give thanks for the blessings of Allah, rather they are arrogant towards Allah and His creation.
12) When Muslims help those Muslims who have been afflicted by calamity, they will be rewarded for that.
The Prophet (pbuh) said: The likeness of the believers in their mutual love, mercy and compassion is that of the body; when one part of it suffers, the rest of the body joins it in staying awake and suffering fever. (Bukhaari, Muslim)