It is explained later in the linked tutorial, on page 15.
The listed reasons to apply this encoding to USB 3.0 are:
- clock recovery - without the encoding, long streams of 1s or 0s would look like DC
- error detection - when the receiver decodes the 10 bit symbol, there are many impossible symbols; receiving one of them signals an error.
A colleague has another hypothesis. Paraphrasing:
The cable has inductance and capacitance. This smooths the attacking front of the pulses. Thus, transmitting a single 1 would leave the bus in a certain electrical state, while transmitting several 1s will leave it in a different electrical state (closer to steady-state). Consequently, sending a 0 afterwords will be at a different potential in the two scenarios.
Thirdly, here is a quote from the standard, section 3.2.1
The receiver needs enough transitions to reliably recover clock and data from the bit stream. To
assure that adequate transitions occur in the bit stream independent of the data content being
transmitted, the transmitter encodes data and control characters into symbols using an 8b/10b code.