Why indeed. This is a real nagging problem I have with Deism: why would a deity go to all the trouble of creating a universe and apparently directly creating life on earth and then take no further interest in it? Why create at all?
One possibility is that the deity is only concerned with playful creativity for its own sake. Creation gives the deity some joy, like an artists developing a work of art, for its 'intrinsic value'. Perhaps there are many worlds that the deity has created in the great universe.
Another possibility is that the deity simply wants to 'know' and is like the mythic 'all seeing eye': the divine is the ultimate observer and is able to know and experience through seeing the actions of human beings, indeed perhaps all or some human consciousness is open to the divine mind from the 'divine side', so that the divine can experience and know all that humans or any other creature can experience and know; this is not just a knowledge of factual information but knowledge by the deity of our subjective experience itself i.e. what it feels like to be us, such that experience becomes also the divine experience.
A further possibility is that the deity seeds worlds with life, in the expectation that some of them will develop, evolve sentient rational beings. As suggested in other posts, the deity may be 'testing' its creation - which is the 'vale of soul making' to find those beings who can ultimately ascend to the same realm or 'state of being' as the deity and continue in eternal good relationship even as divine servants. Perhaps those chosen by the divine are granted this honour once they depart the physical world at death. Or perhaps the divine is seeking our gradual evolution toward a new kind of race of mankind who will find such connection, such union, with the divine that they become the very incarnations of deity.
Another possibility is that the deity simply wants to reproduce itself through the evolution of sentient beings, for the sheer joy of parenthood.