Character pointer assignment - c

Question

This here the code for the program which counts the number of occurrences of all the words in some input . Taken from the book K and R. I pretty much understood everything except why is the author using strdup() anyway.Why can't we just assign(in the function addtree) p->word=w . In the structure tnode, word is clearly a pointer to a character and argument of addtree function is a character pointer.
#include <stdio.h>
#include <ctype.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#define MAXWORD 100
#define BUFSIZE 100
struct tnode { /* the tree node: */
char *word; /* points to the text */
int count; /* number of occurrences */
struct tnode *left; /* left child */
struct tnode *right; /* right child */
};
struct tnode *addtree(struct tnode *, char *);
struct tnode *talloc(void);
void treeprint(struct tnode *);
void ungetch(int);
int getword(char *, int);
int getch(void);
char buf[BUFSIZE]; /* buffer for ungetch */
int bufp = 0; /* next free position in buf */
int main(void) { /* word frequency count */
struct tnode *root;
char word[MAXWORD];
root = NULL;
while(getword(word, MAXWORD) != EOF)
if(isalpha(word[0]))
root = addtree(root, word);
treeprint(root);
exit(0);
}
/* getword: get next word or character from input */
int getword(char *word, int lim) {
int c, getch(void);
void ungetch(int);
char *w = word;
while(isspace(c = getch()))
;
if(c != EOF)
*w++ = c;
if(!isalpha(c)) {
*w = '\0';
return c;
}
for(; --lim > 0; w++)
if(!isalnum(*w = getch())) {
ungetch(*w);
break;
}
*w = '\0';
return word[0];
}
/* addtree: add a node with w, at or below p */
struct tnode *addtree(struct tnode *p, char *w) {
int cond;
if(p == NULL) { /* a new word has arrived */
p = talloc(); /* make a new node */
p->word = strdup(w);
p->count = 1;
p->left = p->right = NULL;
} else if((cond = strcmp(w, p->word)) == 0)
p->count++; /* repeated word */
else if(cond < 0) /* less than into left subtree */
p->left = addtree(p->left, w);
else /* greater than into right subtree */
p->right = addtree(p->right, w);
return p;
}
/* talloc: make a tnode */
struct tnode *talloc(void) {
return(struct tnode *)malloc(sizeof(struct tnode));
}
/* treeprint: in-order print of tree p */
void treeprint(struct tnode *p) {
if(p != NULL) {
treeprint(p->left);
printf("%4d %s\n", p->count, p->word);
treeprint(p->right);
}
}
int getch(void) {
return (bufp > 0) ? buf[--bufp] : getchar();
}
void ungetch(int c) {
if(bufp >= BUFSIZE)
printf("ungetch: too many characters\n");
else
buf[bufp++] = c;
}

Answer 1

Because without strdup you just assign the address of the variable so both will point to the same memory (thus same data).
char *ptr2 = ptr1;
+----------+ +---------+
| PTR1 |---------->| VALUE |
+----------+ +---------+
^
+----------+ |
| PTR2 |----------------+
+----------+
While with strdup a new block of memory is allocated and the characters are copied into new one:
char *ptr2 = strdup(ptr1);
+----------+ +---------+
| PTR1 |---------->| VALUE |
+----------+ +---------+
+----------+ +---------+
| PTR2 |---------->| VALUE |
+----------+ +---------+
You see the difference?

Answer 2

#DCoder is right, which is really the answer hence this post.
The line where the word is first allocated is:
char word[MAXWORD];
The program keeps reeding into this very memory over and over again. Without copying this data for each node, they'd all point to this buffer.