SImple explain of the atof function ???

Just a simple questions about atof function?
Why and when would you need a function that convert char to double value???
What is the purpose??
Can you explain to me the following 2 programs
give me some situations when such a function need to be use.
thanks
 
 str = "  3336402735171707160320 ";  
 
Is "333640273......"  being treated as a string and not a value?
 
* atof example: sine calculator */
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
int main ()
{
double n,m;
double pi=3.1415926535;
char sizeInput [256];
printf ( "Enter degrees: " );
gets_s ( sizeInput );       // WHY does this read in a degree value but declare as char to read up to 256 ???
n = atof ( sizeInput );
m = sin (n*pi/180);
printf ( "The sine of %f degrees is %f\n" , n, m );
return 0;
}
 
 
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// crt_atof.c
//
// This program shows how numbers stored as
// strings can be converted to numeric
// values using the atof function.

#include <stdlib.h>
#include <stdio.h>

int main( void )
{
    char    *str = NULL;
    double  value = 0;

    // An example of the atof function
    // using leading and training spaces.
    str = "  3336402735171707160320 ";
    value = atof( str );
    printf( "Function: atof( \"%s\" ) = %e\n", str, value );

    // Another example of the atof function
    // using the 'd' exponential formatting keyword.
    str = "3.1412764583d210";
    value = atof( str );
    printf( "Function: atof( \"%s\" ) = %e\n", str, value );

    // An example of the atof function
    // using the 'e' exponential formatting keyword.
    str = "  -2309.12E-15";
    value = atof( str );
    printf( "Function: atof( \"%s\" ) = %e\n", str, value );