If you just return an actual List<T> as an IReadOnlyList<T>, then the caller can always just cast it back, and then modify the list as they please. Conversely, calling AsReadOnly() creates a read-only wrapper of the list, which consumers can’t update.
Note that the read-only wrapper will reflect changes made to the underlying list, so code with access to the original list can still update it with the knowledge that any consumers of the read-only version will see those changes.