It makes a lot of technical sense to solder SSDs to the board given the insane speed of modern SSDs. Solder joints are much better than a connector for a high bandwidth link. Even tiny parasitic capacitances become a major headache.
Are they accomplishing anything that couldn't and isn't already done well be an NVMe M.2 connector? All that comes to my mind is a miniscule space savings.
I suspect they are getting better speed and power consumption, and deeper integration with T2 (which functions as the controller chip). Apple is generally ahead of the curve when it comes to SSD performance in laptops, so it's unlikely that a generic SSD connector would function as well.