They probably get used to code in Java, hence they use OOP/struct in C++.

Don't know about them but i personally use this so that in multiple tc's questions i can write

for(...){
    if(xyz){
        cout<<-1<<endl;
        return;
    }
}

Instead of

bool flg=true;
for(...){
    if(xyz){
        flg=false;
        break;
    }
}
if(!flg){
    cout<<-1<<endl;
    continue;
}

Ability to reset global variables at tasks with multitests

One advantage is that order of function defintions doesn't matter. You can use a function defined below your main function inside main function.

struct Solver {
    void mainFunction() {
         f();
    }
    void f() {
       ...
    }
};