Why now a days company start selecting the candidates thru agencies or consultancies ?

save there time and to recruitpples ,which are capable of doing desired job function

Because normally companies themselve have requisite experties to test/decide about the type of qualification they want. 

And also may be companies dun want to spend too much on their HR departments

I think the logic behind this is that there are good number of so called potential candidates but the work can be simplified by agencies as the candidates get screened several times before they finally reach to the concerned company.