The Raise is split into 3 main pieces; the 2 sides and the Neuron.
The Neuron is the brain of the keyboard; the microprocessor and memory are inside it.
It is important because, with it, we are able to remove the left half (of the keyboard), and have the right half keep working or vice versa. That is mainly why we need it as an external piece.
All your settings for the lighting and the keymaps are stored in the Neuron's memory.