For older/vintage cards I don’t know if it’s that important, but for modern sets it at least gives some verified indication of the condition of the card. I’ve heard PSA subscribers talk about wanting them as well.
CGC in general is priced lower because they are the new kid on the block, and they also grade harder than PSA (somehow a harder grade/better quality card is worth less, not sure how but oh well). I’ve ordered a couple cards with sub grade just to give myself a benchmark of how the company grades, but 80-90% of all my orders through CGC are just going to be no sub grade for personal collection.
There may be no real price difference on a card with and without a sub grade, but if that’s the case then it’s a nice added on feature you don’t have to pay extra for.