In low light, you need to use wide aperture because it allows you to use a faster speed, thereby lessening the risk of blur due to camera shake.
A wide aperture also lets you choose a lower ISO setting that usually results in higher image quality. Plus, the narrow depth of field of a wide aperture setting can draw attention to your subject by blurring the foreground and background.
There are drawbacks, however, in using a wide aperture, and these are explained by Mason Resnick in Adorama’s 100 in 100 Part 2, a collection of practical photography tips which is now on its second series.
[Via: Adorama.com]