The use of Index in Access

Hi
I only know to create single-field index and multi-field index to prevent duplicate entries from entering into the Access database. In fact, besides that, how can an Index speed up the sorting of a table ? In Access, we can also do sorting without creating an Index, any evident benefit after creating an index on a field (Besides Uniqueness of the field)?

In Visual Foxpro, after creating an Index, we can c the effect by typing

Set Order To TeacherIndex
List

Thanx in advance.