DJO nailed the safety aspect. There's also the casting angle. With a fixed anchor point, you may not be able to comfortably cast to the spot you want. Altering the tie-point swings the kayak to any angle you choose. That also helps stop the "weaving" that occurs with a single anchor.
I use my trolley with a drift sock, also. Same reasons apply.
EZ-Dock modular docks