Why use iPhoto instead of manually saving images?
Most things you can do in iPhoto you can do in Preview and Finder, except a lot slower. Organization in iPhoto is more powerful than simply using a file system.

You can sort:

by Event
by Location
by Person via facial recognition


Arrange into albums (Unlike folders, photos can reside within multiple albums.)
Shows extra metadata, such as:

Stars
Camera and settings used
Map position


Allows basic image touch ups:

Color-grading
Auto touchup (really hand, fast and convenient)
Built in &quot;straighten&quot; function (also very useful)
Auto-enhance
Automatically retains &quot;originals&quot; in case you want to revert an edit


Manages Facebook and Flickr albums