It is destabilising in roll. But, significant wingsweep (essential in a transonic or supersonic aircraft) is stabilising in roll. It is possible to have too much lateral stability (for a start, it starts rolling all over the place in turbulence, but also it makes it very sluggish in roll), so anhedral is introduced to destabilise. Roll stability will still be positive, but less so.
It also means for aircraft like a Harrier, that the undercarriage legs can be shorter, thus saving weight.
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