While the song is playing the tempo value changes, but I don't hear any change in playback.

The question seems to be why are you not hearing the a Tempo change... I guess it will depend on how subtle or severe the tempo change is... if the instrument is changing tempo and you are not hearing it, we have to ask what tempo values did you record? When you look at the Tempo Track do you see the tempo values written to the Track? what are they? Can you provide more info, recognizing you did not post an audio clip and we are totally dependent on you for the information.

Let us know

How can I use Tempo Track musically

... on the broader subject... of using Tempo Track musically, this should serve as a tutorial:
When it comes to TEMPO CHANGE there are a couple of ways.

You can have the Motif XF create a Tempo Change curve for you automatically. Or you can record it in real time. Or you can automate Tempo Change with SCENE Events.

XF creates a Tempo Change
From your SONG, press [EDIT]
Press [F4] TRACK SEL (Track Select), this is a three-way toggle between the numbered TRACKS, the SCENE TRACK and the TEMPO TRACK. Press it until the TEMPO Track is selected (the track type will appear in the upper right corner).

With the TEMPO TRACK selected:
Press [JOB]
Press [F3] EVENT
Select JOB 05: CREATE CONTINUOUS DATA
Press [ENTER]

Because you selected the Tempo Track the ‘Create Continuous Data” JOB will be set to insert Tempo Change events into the dedicated Track. Set the Measure range to match the exact point you would like the tempo to begin changing, and where you want the change to stop.

Fill in the parameters as you desire. The “Clock” parameter will determine how often the Tempo Change events are inserted. And you can tailor how fast the tempo change occurs.

See Motif XF Reference Manual, page 160)

When to use: You would use this method when you want automate gradual tempo changes. You can set a start and end point, and you can even control how often a tempo event is placed. Trial and error works here - do not be afraid to experiment. While working in the [JOB] area, button [F1] is marked UNDO/REDO and allows you to compare your data at the touch of the button.

Recording TEMPO CHANGE in real time
Call up your SONG
Press [RECORD]
Set the REC TRACK = TEMPO
You can do so on the Setup screen, by cursoring to the “REC TRACK” parameter column and use the [UP CURSOR] to select the TEMPO track.

Now move the highlight to the TEMPO parameter shown in BPM (Beats Per Minute).
You can record yourself changing the tempo in real time via the DATA WHEEL movement.

When to use: You would use this method when you want to manually control the tempo change. Again the JOB > [F1] UNDO/REDO will allow you to experiment without consequence.

Insert TEMPO CHANGES via the SCENE memory
A Motif-series “SCENE” (adopted from Yamaha digital mixing consoles) is a ‘snapshot’ of many parameters that can be instantly recalled by pressing a SCENE button or can be automated with a special SCENE EVENT for recall. This would be excellent when you need aburpt/specific changes in tempos

_ You record a SCENE by documenting the current status. Set the Tempo as desired. HOLD [SHIFT] + [SF1] to take a snapshot.
_ You can store 5 different TEMPOs to the [SF1]-[SF5] buttons.
_ You can recall a SCENE at any time by pressing the [SF1]-[SF5] buttons when they are labeled “SCENE1”. A SCENE containing data will have an eighth icon.
_ You can INSERT or record in real time a SCENE EVENT.

When to use: You would use this method when you manually want to control the tempo by recalling a pre-determined setting.

Tempo Change in PATTERN mode
By design, all Sections of PATTERN mode are the same Tempo. You can automate TEMPO changes by placing the Sections in a playback order, and then entering Tempo change events on a dedicated track for these type of events… this “playback order” is called a “Chain”

From PATTERN mode
Press [F5] CHAIN
You can record yourself ‘directing’ the playback order by placing the sequencer in real time record. The CHAIN will document when you press the SECTION buttons - thus creating an automated playback order.

Alternatively, you can enter the play order by:
Press [F5] CHAIN
Press [EDIT]
Now you can “INSERT” SECTION data on an Event List… you have a list of measures and a selection of EVENTS that you can ENTER, including SECTION CHANGE, Time Signature change, Tempo Change, Track Mute Status.

TEMPO event can be enter at any measure/beat/clock location.

Because of the cycling nature of PATTERN sections it make sense to insert certain changes to the playback of these sections only have you have mapped the playback order.

Say you have an 8 measure SECTION A… and an 8 measure SECTION B. Your compositions structure is going to be the ever poplular AABA song structure… but the second time through you want to increase the tempo… logically speaking, it only makes sense to insert a TEMPO change on a linear time line. That is how the CHAIN works - it gives you a linear time line onto which you can now map out exactly when a change is to take place.

ALTERNATE METHOD:
That said, Tempo changes can be stored to PATTERN SCENES. On the main PATTERN screen you will see SCENE 1 - SCENE 5 on the [SF1]-[SF5] buttons. A Scene is a snapshot of the current condition of the PATTERN. One of the events that can be stored to a SCENE is the tempo. So you can store up to 5 different Tempi, one per [SF] button.

To store a SCENE simply hold [STORE] + [SF]
Store the original Tempo to [SF1]
Store the second tempo to [SF2]

To recall a SCENE simply press the [SF] button.