 Re: also
I think it's also because "also" disambiguates the relative clause, pointing out that it is defining rather than non-defining. Compare :
More than 20,000 people died or were reported missing as a result of the disaster in March which crippled nuclear reactors in the Fukushima area.
Like that - no comma and no "also" - the relative clause becomes defining : it's telling you which disaster in March we're talking about.
With the comma/also it is, as emile says, just giving extra info about a disaster which you are presumed to have already identified. The "also" signposts this and thus disambiguates the clause function. |