Dear Muhammad,
Wavelength conversion is important. Simply it would be used in wavelength division multiplexing network where you need multiple carriers to be used. Also, a certain wavelength might be needed for a specific purpose, while the laser/LED source available emitting at different wavelength. Many examples, like for optical communications, 1550 nm and 1300nm are preferred for minimum attenuation and dispersion respectively!! while for medical application like imaging, around 1 um wavelength is used to be absorbed by human body. Super-continuum generation is needed for different kind of imaging systems!
Hope that clears up the need for wavelength conversion. and many other applications could be found in the literatures.
Regards
Mohamed Abouseif
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