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!