Once upon a time not long ago, high fashion brands used to distance themselves from all fast fashion through creativity and the uniqueness in their designs. Nowadays, HF desperately chasing after FF with all these pointless seasons that served no purpose beside make more money. Pre-spring / Resort, Pre-Fall, what next Pre-Summer, Pre-Winter. Seriously do you need to buy expensive clothes that have few tweaks here and there every 2 months.
I think the actual use of a pre-collection heavily on the brand.
For Brand A, it could a extra marketing opportunity by showing in a foreign country or a famous landmark.
For Brand B, it could be the way they are financially successful, despite presenting conceptual collections at Fashion Week.
For Brand C, it's a transparent venture to sell more clothes, more often without making too much effort.
For Brand D, it might be the only type of collection they make, because buyers are allocated larger budgets for those collections and they last longer in stores.
For YSL, it's most likely a mix of B and C. Most of the collection that was presented in September (like its predecessors) won't even go into production. What is produced are the bags and the shoes (with several modifications made to avoid permanently crippling anyone who as much as glances at them). This is a safer, more secure sell.
And the slow, but inevitable end of the high fashion/fast fashion divide is due to the high fashion industry trying to appeal to a generation that specialises in "dismantling", "disrupting" and "eating the rich" AKA a generation that wants high fashion dead and gone. The line is also blurred by the rise of Instagram and digitally native brands that sell for more than fast fashion, but less than high fashion.
It's all just a tangled mess, really.