The reason for asking for funds is for people who haven't yet arrived in Canada and may not have arranged a job in Canada. They want to make sure that they have enough money to support themselves and get settled when they arrive and until they find a job and start to get paid.
This is actually in the proof of funds section for the skilled worker class, see
http://www.cic.gc.ca/english/immigrate/skilled/funds.aspYou do not have to show that you have these funds if you have arranged employment in Canada.
If FSW's get out of showing funds if they have arranged employment, I am sure PNP applicants who are already living and working in Canada do not need to show funds either.