Choose your analogy:
You can build a reef tank with just water changes. (No CO2)
You can manually dose elements in the reef (DIY CO2)
You can get a calcium reactor. (Pressurized CO2)
You can off road in a 2wd (No CO2)
You can off road in a 4wd (DIY CO2)
You can off road in a 4wd with a wench, proper tires, experience (Pressurized CO2)
You can slice potatoes with a knife
You can slice potatoes with a slicer
You can slice potatoes with a food processor
Is it required, no, but it makes life easier once tuned. Pressurized CO2 is like a 10x health cheat code in a game. There is still a chance you die, but it is much easier. But depending on your tank/budget, there is a significant or minimal cost incurred. If you can assume the cost, I recommend it. If you can't, don't feel bad. You can still accomplish a great tank, but it will require more work and patience and ...
Here is one of my no CO2
08132020 - today
Ignore the reflection, just took a quick pic and have a reef on the other side of the room. Didnt setup the blackout cloth holders.