C program to convert a decimal number to binary string

I was given this program to convert a inputted decimal number to a string which is binary number. Now this is a correct program and does work. I have a doubt in while loop the expression saying ch = (rem + 48); I believe value of remainder rem is ultimately going to be inherited by ch then why adding 0 ie. 48 in ascii terms is making difference. what is making addition of 48 convert rem to character form. If I just write ch = rem;, ch is not considered as character and adding 48 makes it charcter. But Why???

void main()
{
    char x[15],tmp,ch;
    int i=0,j=0,dno,rem;
    printf("\nEnter decimal number:");
    scanf("%d",&dno);
    while(dno>0)
    {
        rem = dno % 2;
        ch = (rem+48);
        x[j++] = ch;
        dno/=2;
    }
    x[j--]='\0';
    while(i<j)
    {
        tmp = x[j];
        x[j--] = x[i];
        x[i++] = tmp;
    }
    puts(x);
    return;
}