In my view the answer has two aspects.
One is to draw attention to the magnificence of Allah who has created these four things three of which are fruits whose usefulness for mankind is limitless. fourth is city and mention of Mecca is prophetic. This surah was revealed in Mecca when there was no peace for Muslims in it, but once Mecca came under control of Muslims, it has been an abode of peace. There had been some instances where due to some internal strife, there was a breach of peace but this was minor compared to Jerusalem (which also means city of peace in Hebrew) which was named so by Jews themselves.
As prophesied by the Prophet (saws) in certain Ahadith it will always remain a symbol of peace.
Second explanation is as these four things are related to four great prophets mentioned in Qur'an so believers are reminded that as those incidences related to those prophets have been fulfilled so what is being said in this surah will also be fulfilled. Fig is related to Adam (as)- he used fig leaves to cover his nakedness. olive is related to Noah (as), when he released dove it came back with an olive branch meaning that there is place to land the boat somewhere. Mecca is related to Abraham(as) and Mohammad(saws) it has been a city of peace, though for some time it was not for the greatest prophet of all time (saws) but ultimately it was given to Muslims. One finds such refs in bible as well like in Duet 33:2 , Gen 3:7 & 8:11