Why are cdna libraries considered so valuable?

CDNA does not have introns and therefore can be expressed in prokaryotic cells. cDNA libraries are most useful in reverse genetics where the additional genomic information is less use. Also, it is useful for subsequently isolating the gene that codes for that mRNA.

Here, first we need to understand the meaning of cDNA library. It is a combination of cloned cDNA (complementary DNA) fragments inserted into a collection of host cells, which together constitute some portion of the transcriptome of the organism and stored as a "library".

As that portion (product which we produced) have specific, organised information, the way in which we proceed, it can work as a bank of that code in the future, so it is valuable for us

Hope this helps!