Answer
Believing in God is necessary because sooner or later we are going to face Him SWT. No doubt He SWT is present all the time but “That day” He SWT will present Himself in front of us and will judge us on the basis of our deeds. This is an undeniable fact and the Prophets(A) came with the same message which revolved around the exact same thing that one day you are going to meet your Lord and He SWT is going to ask you things so prepare yourself for that day. Let’s try to understand this with an example from our daily life. Human beings always tend to plan things ahead of time. If you have children, you start saving for their future/education etc. If you are travelling, you look at the weather forecast before packing so that you are well packed according to the changing temperatures and weather.
Allah SWT is for REAL and we will be facing Him. Facing the Lord of the universe would be an extremely difficult time for humanity. The Prophets (A) came to tell us to prepare ourselves ahead of time otherwise we will be all messed up. For that reason our God has sent us constant and consistent reminders so that we are well aware of that “meeting” with our Lord. The awareness of our Lord and Meeting with Him SWT is basically the reason why we need to believe in our Lord.
If you are interested in understanding the rational arguments that support this answer, you can do a detailed course that I taught called “Wajood e bari Taalah” [The Existence of God] and inshallah it will clear all your doubts.