She is the fifth maid character to appear in the series. The other maid characters are Ruukoto, Mugetsu, Yumeko, and Alice. She also bears a strong resemblence to Yumeko, since both are blade-throwing maids who serve demon-like masters. On a related note, Mugetsu is not truly a maid since she was just cosplaying. Although not many know it, Alice became Mima's maid in Mystic Square.
According to Sakuya, if she happens to run out of knives, she can freeze time and pick up the ones she threw previously.
According to ZUN, "Manipulation of Time" is synonymous to "Manipulation of Space." She takes advantage of this fact in several ways, from enlarging the space inside Scarlet Devil Mansion, to probably storing an immense amount of knives around. Maybe it's general relativity?
Sakuya can also use her powers to accelerate time, such as making bamboo flowers bloom instantly, even though this would normally take 60 years to grow.
Sakuya may be an homage to Dio Brando, the main villain of the manga series JoJo's Bizarre Adventure, a vampire who is known for stopping time and being skilled at throwing knives. JoJo's Bizarre Adventure has been referenced several times in the official games.
Sakuya "prepares" food for Remilia and Flandre Scarlet. It's a mystery how this is done exactly, although she mentions in Phantasmagoria of Flower View that with so many flowers around, she can make as much tea and as many pastries as she wants. (Flowers have often been used to provide flavoring for tea, especially in the past, and are still the basis of jasmine tea.)
There have been several clues spread across the Touhou series that suggest that Sakuya may not actually be human. Most of these occur in Imperishable Night, where Eirin Yagokoro is surprised to see her (according to Eirin's profile). Reimu also questions her humanity in Embodiment of Scarlet Devil.