[R-meta] The role of 'W' argument in the rma.mv function

Dear all,

Could you provide an explanation or an example on why 'W' (for rma.mv) or
'weights' (for rma.uni) should be applied in a meta-analytic model? If I
understand correctly, one should in principle "weight" the observation by
the inverse of sampling variance. Is then possible to specify additional
"weight" to the observations in the model (assuming this is what 'W' does)?
If yes, when does it makes sense to use it?

Thanks,
Gabri