"its only purpose" is correct and usable in written English.
You can use it when describing the singular or primary reason or intention for something. Example: The function of a hammer is to drive nails into wood, but its only purpose is to provide a means for constructing sturdy structures.
My work is comic but it's not its only purpose.
You know it's designer when its only purpose is to look beautiful, and this little chippy owl is certainly beautiful, despite the shiny red ornament it once was. 5. Footed Floral Platter Project via Hometalker Jeanie @Create & Babble.
It is unusual in that its only purpose is to guide readers to other Web sites.
Gears never has, though – every time you pick up a new instrument of destruction, it immediately feels right, like its only purpose is to send death to enemies, whatever their size or scale of swarm.
It also keeps your proxy network from looking as if its only purpose is to make money.
Many users today download the new Nearby Live thinking it's another dating app, but Hamachek clarifies that's not its only purpose.
Facebook insists it is preserving user privacy by neutering their API for its only stated purpose: "[E]nabling users to share their information with the third party websites and applications they choose".
As such, one could claim that its only real purpose is to rapidly cycle low-enriched uranium stocks into highly-enriched uranium usable in a nuclear weapon.
Reading the agreement, you get the feeling that its only real purpose has been to cement the union's position in the workplace.
A carnelian vase in the form of a gourd could hardly be less vaselike; its only real purpose is to draw attention to the ingenious way the artist has turned a natural pattern of light-color veins into a network of vines.
But, then, his was a costume drama whose only purpose was to remove its costume.
