what is the purpose of FOREACH

A drawback of foreach compared to an ordinary 'for' loop is that it can only iterate through an enumerable collection in a forward direction.

However, you can get around this by using the LINQ extension method Reverse() to reverse the order of the collection.

As it's doing more behind the scenes, foreach is also slightly slower than for, though the difference is unimportant in the vast majority of applications. 

Foreach is a loop construct. It does not use an integer index. Instead, it is used on a collection and returns each element in order. This is called enumeration. It eliminates errors caused by incorrect index handling.

ex:-
using System;

class Program
{
    static void Main()
    {
    // Use a string array to loop over.
    string[] ferns =
    {
        "Psilotopsida",
        "Equisetopsida",
        "Marattiopsida",
        "Polypodiopsida"
    };
    // Loop with the foreach keyword.
    foreach (string value in ferns)
    {
        Console.WriteLine(value);
    }
 Console.ReadLine();

    }
}