It is common to see loops that immediately compute a value from their iteration variable and then never use the iteration variable again in the rest of the loop (see example below). The intent of such loops is arguably not to iterate over the original sequence at all, but to iterate over the sequence that results from transforming the original sequence in some manner.

There is a good case to be made that the code is more readable if this intent is expressed explicitly, which can be done by using LINQ to perform a Select on the input sequence. The resulting code is clearer due to better separation of concerns.

This example iterates over a list of i².

This could be better expressed by using LINQ's Select method with a lambda expression.

MSDN: Enumerable.Select Method.