为什么结构体一般定义成指针变量？

#include<stdlib.h>
#include<string.h>
#define MAX 16
typedef struct
{
    int data[MAX];
    int length;
}SqList;
void init(SqList *L)
{
    SqList *L2;
    int i = 0;
    L2 = (SqList *)malloc(sizeof(SqList));
    L2->length = 10;
    for (; i < L2->length; i++)
    {
        L2->data[i] = 97;
    }
    *L = *L2;

}
void display(SqList L)
{
    int i = 0;
    for (; i<L.length; i++){
        printf("%d ", L.data[i]);
    }

}
//******************************************
//参数传入为指针变量的地址
void init2(SqList **L)
{
    int i = 0;
    // 返回分配给结构的首地址，把它存储到指针变量中
    *L = (SqList *)malloc(sizeof(SqList));
    (*L)->length = 10;
    for (; i < (**L).length; i++)
    {
        (*L)->data[i] = 12;
    }
    

}

void display2(SqList *L)
{
    int i = 0;
    for (; i<L->length; i++){
        printf("%d ", L->data[i]);
    }

}

int _tmain(int argc, _TCHAR* argv[])
{
    SqList L;
    init(&L);
    display(L);
    printf("\n");
    SqList* pL;
    init2(&pL);
    display2(pL);

    return 0;
}

    SqList* pL;
    init2(&pL);
    display2(pL);