If Allah has decreed everything for us, what is the point of supplicating to Him and asking him for something?

What Allah decided is the reward for those who supplicate and the lack of reward for those who do not. He does not decide whether you supplicate or not, you do, but He knows if you will supplicate or you will not.

The Prophet (ﷺ) said in a Hadith:

“Dua is worship.” [1]

Allah says in the Quran:

وَقَالَ رَبُّكُمُ ادْعُونِي أَسْتَجِبْ لَكُمْ إنَّ الَّذِيْنَ يَسْتَكْبِرُوْنَ عَنْ عِبَادَتِيْ سَيَدْخُلُوْنَ جَهَنَّمَ دَاخِرِيْنَ

“And Lord said, ‘Call upon Me and I will respond to you!’ Verily, those who are too arrogant to worship Allah will enter Hell humiliated.” [2]

The entire concept of worship, the purpose for which man was created can be summarized in one simple act of dua, as Allah says in the Quran:

وَمَا خَلَقْتُ الْجِنَّ والْإنْسَ إلاَّ لِيَعْبُدُوْنَ

“And I have not created jinn and man except that they worship Me.” [3]

References:

[1] Athenticated, narrated by Ahmad and the four Sunans, and others, from Nu’man ibn Bashir, as reported in Sahih al-Jami, no. 3407.

[2] Quran, 40:60.

[3] Quran, 50:56.