As a humble biologist, my delvings into this are shaky (that's my massive disclaimer...). It seems that if (theoretically) the telescope were to remain at L2 perfectly, it would experience a permanent annular eclipse of the sun (by the earth), but that is not only a very bad option as far as the telescope's solar panels are concerned, it's not possible anyway because L2 is unstable, which requires constant tweaking of the mission's position to stop it from waltzing off into space. Hence the need for this somewhat wobbly halo orbit...

I never cease to be amazed by this stuff...