Even if determinism isn't true for everything that happens—even if
some things just happen without being determined by causes that were
there in advance—it would still be very significant if everything we
did were determined before we did it. However free you might feel when
'It' is a dummy subject. English clauses which are not imperatives must have a subject. Sometimes we need to use a ‘dummy’ or ‘empty’ or ‘artificial’ subject when there is no subject attached to the verb, and where the real subject is somewhere else in the clause.