Confusion about TF Odom to Base_footprint

edit

So I have some confusion about transformation between odom and base_footprint. I am using turtlebot_gazebo turtlebot3_house,launch. After I ran this launch file, I checked the TF frame view. I find that the only transform is odom->base_footprint.

My understanding is: odom frame and base_footprint should both be the position of robot. Why this tf exist? Can someone explain this? Thanks in advance!

Below is /odom topic

 zhefan@zhefan-OMEN-by-HP-Laptop-17-an0xx:~$ rostopic echo -n1 odom
    header: 
      seq: 4454
      stamp: 
        secs: 148
        nsecs: 500000000
          frame_id: "odom"
        child_frame_id: "base_footprint"
        pose: 
          pose: 
            position: 
              x: -3.0000338659
              y: 1.00008570224
              z: -0.00100139306072
            orientation: 
              x: -9.64725127562e-06
              y: 0.00385309683028
              z: 0.00067114823324
              w: 0.999992351527

And this is tf:

transforms: 
  - 
    header: 
      seq: 0
      stamp: 
        secs: 38
        nsecs: 700000000
      frame_id: "odom"
    child_frame_id: "base_footprint"
    transform: 
      translation: 
        x: -3.00002305648
        y: 1.00002237871
        z: -0.00100139296313
      rotation: 
        x: -7.72369546136e-06
        y: 0.00385310031639
        z: 0.000171801496723
        w: 0.999992561994
---