In C, why do some people cast the pointer before freeing it?

Casting may be required to resolve compiler warnings if the pointers are const. Here is an example of code that causes a warning without casting the argument of free:

const float* velocity = malloc(2*sizeof(float));
free(velocity);

And the compiler (gcc 4.8.3) says:

main.c: In function ‘main’:
main.c:9:5: warning: passing argument 1 of ‘free’ discards ‘const’ qualifier from pointer target type [enabled by default]
     free(velocity);
     ^
In file included from main.c:2:0:
/usr/include/stdlib.h:482:13: note: expected ‘void *’ but argument is of type ‘const float *’
 extern void free (void *__ptr) __THROW;

If you use free((float*) velocity); the compiler stops complaining.