I would conjecture the reason there are 30-day CDs is so you can have very liquid assets. Buying a 30 day CD with cash you dont immediately need would be slightly profitable, because you would recieve  marginal interest.

Of course, the shorter the CD, the less the interest rate, so naturally a 30 day CD is rediculously low. This is better than nothing however.

Would it help your credit? Well it wont hurt it, but wont help a whole lot either.