The "ons" and "offs" of the previous signal are clear. But what if the signal is sent down a long wire and someone nearby turns on a vacuum cleaner? The whirling and sparking electric motor creates a great deal of electrical noise. The graph shows the signal at the other end of the wire.
Even though the signal is noisy (at the analog level), the binary values are transmitted perfectly. You (and the electronics) can still tell that at time T1 the signal represents "off" and that at time T2 the signal represents "on". The receiving end just needs to get the binary values.
Since only the "on" "off" information matters, the analog noise is irrelevant, and the original signal is received perfectly (so far as the binary information goes.)
If the signal were regarded as an analog signal, and exact values were important, would some information present in the first signal (before the noise) have been lost?