The Guardians of Light and Darkness in Time (also known as the Black and White Guardians) appear to be higher Eternals, beings of mind boggling power but, significantly, unwilling to 'be seen' actively changing the universe. Certainly the Eternals of Enlightenment know, and are terrified, of the Guardians.
Their purposes is to maintain the balance of the universe, indicating a form of universal order. Another element in this balance is the Key to Time. It is possible that the fourth Doctor's search for the segments of the Key is a Black Guardian trap, as it is known that both Guardians can appear in any form (another Eternal trait).
The assembled Key stops time itself, allowing the Guardians to alter the 'balance' of the universe. Does the Key work, despite being assembled for only a very short time? The Doctor speculates that harmony has been restored but he is hardly an expert. The events of Logopolis and the approaching heat death suggest that the Doctor may have disassembled the Key too early.