To expand on this a bit more, bits are used for data transmission rates because various types of encoding, padding, and parity means that data on the wire isn't always 8 bits per byte. Dial up modems were very frequently 9 bits per byte (8-n-1 signalling), and for something more modern PCIe uses 8b/10b encoding, which is 10 bits on the line for each 8 bits of actual payload.