useEffect(() => {
        let interval = null;
        if (isActive) {
            document.title = `${seconds} seconds`;

            interval = setInterval(() => {
                setSeconds((seconds) => seconds + 1);
            }, 1000);
        } else if (!isActive && seconds != 0) {
            document.title = `React App`;

            clearInterval(interval);
        }
        return () => clearInterval(interval);
    }, [isActive, seconds]);

Why the interval variable assigned to null in the first line of the useEffect hook?