But its orbit isn't perfect or perfectly stable: Other forces than just the Sun-Earth system act on it, albeit very weakly. Some include photonic pressure (phot

But its orbit isn't perfect or perfectly stable: Other forces than just the Sun-Earth system act on it, albeit very weakly. Some include photonic pressure (photons from the sun pushing on one side, not the other), the moon, a few scattered particles (space isn't perfectly empty), etc. And the fact that the JWST itself isn't a point mass means that it is tugged at by gravity slightly differently in different places. Then add to all of these the most important fact that L2 is an unstable equilibrium (a local potential maximum, like a hill), and the telescope is more like a dancer on its toes needing to right itself by throwing something every now and then. All these things necessitate expending some fuel for station keeping.