Why define struct in variable?

Consider the following program:

Many people when they want to use a struct, they create a new variable as:

struct structureName variableName

While it works when you just define it as:

 structureName variableName

My teacher always uses the first method. My question is how do they differ? Do I ever need to specify struct before defining my variableName. Here is an example to explain my question:

struct example {
    int n;
    char c;
};

int main() {
    example o;
    o.c = 'c';
    o.n = 5;
    printf("%c", o.c);
    printf("%d\n", o.n); //this works

    struct example ex; // this versus "example o" without using struct keyword
    ex.c = 'e';
    ex.n = 7;
    printf("%c", ex.c);
    printf("%d", ex.n); //this works

    return 0;
}