What's the relationship between navigation and TF?

edit

I bumped into an error that I cannot do navigation.

The error happened is probably caused by wrong tf transformation.

I want to know what's the relationship between tf and navigation, is tf important to navigation? (I've seen this tutorial, but I don't understand why tf from left arm is important to navigation).

Thanks in advance~

EDIT:(Thanks to Austin)

As a part of the sensor processing for the PR2, the laser scans are filtered to remove any hits on the body of the robot; this is done by comparing laser scan points to the 3D model of the robot in the URDF, and removing any laser hits from the body of the robot. Since the left arm is present in the URDF, the filter tries to remove it from laser scans, but since there is no TF data for the left arm because it is disabled, the filtering step fails and no filtered laser data is published. Since filtered laser data isn't published, move_base doesn't get any sensor data, and refuses to move.