The comma is necessary because the following relative clause is non-essential or descriptive. It is not an essential, or restrictive clause.
Today the Gillette Good News razor dominates the disposable category, (which has grown to dominate the razor blade business).
This razor now dominates the disposable category.
By the way, the disposable category dominates the razor blade business.
Today the Gillette Good News razor dominates the disposable category which has grown to dominate the razor blade business.
There are several disposable categories.
This razor now dominates the particular disposable category that has grown to dominate the razor blade business.