With split keyboards, there needs to be a "central" that maintains state across the keyboard halves and keeps the two synchronized. Without a central, it would not be possible to have basic functionality such as toggling a layer on one half followed by a key on the other half.
Using an external USB dongle as the central ensures the longest battery life for the keyboard halves. In such a setup, the dongle central would receive keyboard data from the halves (peripherals) and handle the main keyboard logic. The dongle subsequently acts as a unified keyboard to the computer that it is plugged into via USB. The dongle can also act as a regular Bluetooth keyboard and additional device can connect to it wirelessly.
The aforementioned setup is very efficient because the keyboard peripherals only need to send data. It can rely on interrupts/events to detect when a key is pressed but sleep whenever it is not in use. A 400mAh battery would last around 9 months.
The dongle central on the other hand must be constantly awake because it doesn't know when the peripheral will send data. The dongle's power consumption is less of a concern because it's either plugged into a computer (for use over both USB and Bluetooth), a power brick (for use over only Bluetooth), or portable charger that has significant capacity (for use over only Bluetooth).
Without a dongle, one of the keyboard halves (typically the left) must act as the central half. That half would use a lot more power and would last around a month on a 400mAh battery. The other half (typically the right) remains a peripheral and would still last around 9 months per charge.