Finally I got the point that why we need "LINQ".
With the help of "LINQ", we can process a list of data in C#, just like what we do in SQL statements. The data in C# is no longer like a list of "single value", but a "stream". Instead of process one hundred variable (which normally stored in a List or an Array variable) one by one, we can process all the data in one go.
I know it's a bit late to understand such a simple concept. Well, better late than never.