Question about nested loops

You use a nested loop for a lot of reasons: Think about it in real life.
When you read a book, you are using a loop:

foreach page in book
   {
   read page
   }

But reading a page involves a loop as well:

foreach line in page
   {
   read line
   }

And to read a line, you need to read each word. and for each word, you need to read each character.
 
Computing is the same: you want to do a task, which needs you to loop through items. Each item needs further processing which requires you to loop though smaller portions.
 

for (i = 0; i < linesCount; i++)
   {
   char* line = lines[i];
   for (j = 0; j < line.Length; j++)
      {
      // Do somethign with each character of the line
      }
   }