How are the frequencies in 50 GHz channels in DWDM networks used?
In WDM (Wavelength Division Multiplexing) networks, the channel frequency given is the nominal optical wavelength.  DWDM (Dense WDM) the channels are more closely spaced, to allow more channels.  DWDM requires more stable and precise laser wavelength control and narrower accurate receive filters.

To fit more optical channels between the existing DWDM channels becomes more expensive and technically difficult.  Also, as the channels become narrower, at some point you have to reduce the modulated data rate or change the modulation method.

This is all a cost / difficulty / need / standardization and interoperability tradeoff.