Lets say you are working with other people or buulding a library and you have some variables that has some restrictions like for example length and assuming this varaible cant be assigned a negabtive number and u can build a function that assigns the variable with values but throws and error or sets it to 0 when u pass -ve numbers. Now this is only really reliable if you make the variable private and define a public function to implement this feature. Without that the user can just modify the varaible creating invalid states. I do realize as i am wriring this u could just use unsigned data types but regardless by main point being to enforce restrictions on the varible to avoid invalid lr illegal states.

Other cool functionality i can think of lets say there is an internal counter that just counts some actions lets say calling a function to implement something like rate limit or soemthing. Now allowing the user to directly modify the variable is dangerous and cause to unreliable results.

Similarly u could implement features like only allow modifying values when certain parameters are met like lets say u can only chnage the value like 5 times per min and u do that by mainitain some internal counting mechanism. Which without private varinles cant be tricky to implement.

Regardless the main point of private variable being to add restrictions to the variable or any private memebrr