When to use free() in C ? when to release the heap memory and when not ?

Quote:

When to use the free() method in C. Below I have stated a Linkedlist , the program is fine evn without releasing the memory here ? What is happening here ?
When should I use the free() method or release the memory from the heap ?

#include "stdio.h"
#include "stdlib.h"

struct Node {
    int data;
    struct Node *next;
};

struct Node *head = NULL;

void insert(int data) {
    
    struct Node *newNode = malloc(sizeof(struct Node));
    
    if (head == NULL) {
        head = newNode;
        head->data = data;
        head->next = NULL;
    } 
    else {
        
        struct Node *temp = head;
        while (temp->next != NULL) {
            temp = temp->next;
        }
        temp->next = newNode;
        newNode->data = data;
        newNode->next = NULL;
    }
}

void display() {
    
    if (head == NULL) {
        
        printf("LIST IS EMPTY!");
    } else {
        struct Node *temp = head;
        while (temp != NULL) {
            printf("%d\n", temp->data);
            temp = temp->next;
        }
    }
}

int main() {
    insert(11);
    insert(12);
    insert(13);
    insert(14);
    display();
}