Why gmapping needs base_link → odom ?

edit

In my idea, the frame should be:

map->odom->base link->laser

gmapping says it needs:

<the frame attached to incoming scans> → base_link
    usually a fixed value, broadcast periodically by a robot_state_publisher, or a tf static_transform_publisher.
base_link → odom
    usually provided by the odometry system (e.g., the driver for the mobile base)

Why it just reverse?

Am I missing something?

Thank you~