Each punch card was it's own line of text.

By putting the final curly brace on it's own card, and hence line, it meant you could add lines to blocks without having to change the old last line.

E.g. the following code meant you only had to type a new card and insert it.

     for(i=0;i<10;i++){         /* Card 1 */
          printf("%d ", i);     /* Card 2 */
     }                          /* Card 3 */

     for(i=0;i<10;i++){         /* Card 1 */
          printf("%d ", i);     /* Card 2 */
          printf("%d\n", i*i);  /* Card 3 */
     }                          /* Card 4 */

     for(i=0;i<10;i++){         /* Card 1 */
          printf("%d ", i);}    /* Card 2 */
     
     for(i=0;i<10;i++){         /* Card 1 */
          printf("%d ", i);     /* Card 2' */
          printf("%d\n", i*i);} /* Card 3 */

I'm dubious of this explanation because C itself largely postdates punched cards as a major medium of data storage, and some quick searches doesn't produce any evidence of people using punch cards with C or Unix.