Why generally EDFA and Raman are used in conjunction to each other?

Counter pump distributed Raman amplifiers are often combined with EDFA pre-amps to extend span distances. This hybrid configuration can provide 6 dB improvement in the OSNR, which can significantly extend span lengths or increase span loss budget. Counter pump Raman Amplifier can also help reduce nonlinear effects by allowing for channel launch power reduction.

Raman Amplifiers are very sensitive to input power so they are always used with EDFA in cascaded fashion. (a small change at input will result in high output power change and thus subsequent components may suffer)