Why do we use cRNA instead of initial extracted RNA in the microarray technique?
After clarification via the comments of the OPs question:

1) The procedure you describe is not for standard RNA microarray experiments, it for olgionucleotide microarray. This type of chip is special because it has multiple probes for each gene to allow detection of special features (i.e. mutations, alternative splicing, ...).

2) The reason for the generation of cDNA with subsequent reverse transcription and biotinylation is not explicitly stated in the source of the OP, but there are some possible advantages:


Amplification of material: cDNA can easily be amplified, which would make it easier to analyse samples whit low RNA yield, genes with low expression levels or to analyse the same sample on multiple chips.
Sensivity: oligonucleotide arrays may use very short (10-25 bases) probes, for which fluorescence readout may not be sensitive enough. Biotinylation of a given RNA base (by the use of pre-biotinylated NTPs during reverse transcription) can give multiple 'marks' on each fragment, which can be read with out streptavidin coupled signals. Additionally this would allow the signal strength to change in respond to certain SNPs/mutations.