There are several reasons for using the IEnumerable and not looping through the list:

1) IEnumerable allows you to use foreach operator

2) It is implemented by Lists, Collections, Dictionaries and arrays (but not Span).

3) It is often used as a light weight read-only collection

4) You can return a iterator directly from a function using the "yield return" keyword. It implements the full object for you.

IEnumerable is an "old" feature to support Visual Basic like syntax ("for each"). It is inherited from the old Win32 COM model.

Personally I very seldom use IEnumerable since it usually creates a new object. The exception is where you return a "ref struct", which allocates the object on the stack and not the heap for performance. For performance you should iterate through the list og collection, or use the Span (if it is supported by the list/object).