What's the point of materia?

[Kalien]

I'd be okay with materia as a way to customize stats on battle gear...even if it's just an 'illusion of choice' thing, I feel that's better than no choice at all.
But, people would complain that it's a gil sink.

I don't understand why people are perfectly ok buying better stats with one type of imaginary currency (tombstones) but not with another (Gil). But yes, there would be whining.