It can be, only if it is necessary.

If we don't care don't want the return value to be used. there is no use storing the same.

If you have an idea how you are going to use x then of course you may write

int x = printf( "text" );

Otherwise the return value of the function is simply discarded because it is not used.

And in most cases programmers do not find a useful application of the return value of printf.

However sometimes it can be used for example to print tables with aligned columns.

For example

int n = printf( "%s", SomeString );

if ( n < COLUMN_WIDTH ) printf( "%*c", COLUMN_WIDTH - n, ' ' );

Consider this simple program

#include <stdio.h>

int main( void ) 
{
    const int COLUMN_WIDTH = 20;

    int n = printf( "%s", "Hello" );
    if ( n < COLUMN_WIDTH ) printf( "%*c", COLUMN_WIDTH - n, ' ' );
    printf( "%s", "World" );
}

Its output is

Hello               World

Here is another example where the return value of printf finds a useful application.

Let's assume that you need to output a sequence of numbers separated with a comma like for example

1, 2, 3, 4, 5, 6, 7, 8, 9

How to output such a sequence using only a single loop without placing print statements outside the loop?

Here is a program that shows how it can be done based on using the return value of the function printf. :) Try it.

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

 int main( void ) 
 { 
    size_t n = 0; 

    printf( "Enter the number of elements in the array (greater than 0): " ); 
    scanf( "%zu", &n ); 

    if ( n == 0 ) exit( 1 ); 

    int a[n]; 

    srand( ( unsigned int )time( NULL ) ); 

    size_t i; 

    for ( i = 0; i < n; i++ ) a[ i ] = rand() % n; 

    i = 0; 

    do 
    { 
        printf( "%d", a[ i ] ); 
    } while ( ++i < n && printf( ", ") > 0 ); 

    return 0; 
 } 

As for your program

int foo(int x) {
  return x;
}
int main() {
  foo(10);
}

then calling function foo does not have any effect. It does nothing and in fact can be removed. And not all compilers issue a message for your program. It seems that the compiler you are using has a compiler option that forse the compiler to consider warnings like errors. So your compiler wants that you would pay the attention to that the call of the function does not have any effect. It is possible that you made a logical error.

On the other hand calling function printf has a visible effect.

You can store the return value if you need it, for example to be sure that you are outputting the desired number of characters (it sounds very unlikely that one has this needs, especially considering that in production you usually have more comprehensive logging modules to handle your outputs).

You can also explicitly say that you don't care about that return value to the compiler with a (void) as in

(void) printf("hello world\n");

Note that this has no implications other than suppressing warnings of some tools (e.g., lint)