Personally - there are multiple reasons and I agree with all of them.

1. Give more options to a kiltwearer, especially those who are dedicated to only one named ancestral tartan. (i.e. there are 3 modern Urquhart tartans, all of which are *basically* a black watch variant with a stripe or two in red or red/white - having ancient/weathered gives new color options without going to a different tartan)

2. Gives a kilted option that has a less "Military" look, and looks well suited to earthtone / country / less formal clothing.

3. Sell more tartan.