In humans it constitute larger part of the genome but did not codes for any protein.what is the need of presence of introns?

This isn’t a simple question ;)

First, there are several kinds of introns. I’m will focus on the group II introns found in eukaryotic genes (See the Wikipedia intron entry Intron - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Intron))

Second, a definition, introns are the regions between exons within a gene. So an intron is defined by what it isn’t. So you can expect that an intron can contain a variety of sites with different purposes (regulatory signals such as enhancers would be an example).

Third, what do we know about introns? Neither the size nor the overall sequence of introns are conserved, but the location of intron-exon boundaries in mRNA are conserved. For the vast majority of introns these sites are conserved in all animals. However, if you look further from humans, in an evolutionary sense , you will find quite a variation in the % of genes with introns.

The need for introns could looked at the level of the gene or a from genomic perspective. From a gene viewpoint an intron is usually needed get the gene expressed. While there are exceptions (called single exon genes & histone genes are an example) most genes require an intron in order for the gene (RNA) transcript to get processed to form a mature mRNA.

From a genomic perspective the presence of introns provides a powerful means for gene to evolve. When asking about the need for introns you need to reflect on the origins of introns. Did they appear early in the evolution of life or later. You can check out these theories at the Wikipedia intron entry.

So, I would argue that the most important “need” for introns is in evolution.