It rank higher, as high PR sites have long age (several years), when they write article their article will get indexed almost immediately. High PR also good for backlinking, putting a link a high PR site and link to your website, produce a lot of link juice for your website, making your website rank higher. You can also sell links in your high PR webpages to make some money.
High PR sites gives you the advantage when you sale advertisement space. because most of advertisers like to advertise in high PR sites only.
High PR sites got an authority from google and Google find them trustable or has high quality content. It is not too hard to have your site got high PR. Just focus on the quality of your site and you may be amazed with the result. If you have high PR sites, you can sell link or ask advertisers to place their ads. They would love to, because the links coming from high PR sites are very high quality and worth to pay.
It's hard to increase page rank of a site for the excessive competition in this field. And the main advantage of your high page ranked site is it gets the most clicks and views.
is back link submission to higher directories and submission only the main reason to increase your PR
Tottaly agree, it's not easy to achieve a high PR so it brings a lot of advantages to do the homework on the long term.
High PR websites help only in indexing. They have a wide impact on rankings as Google Web Spam Team Head Matt Cutts says PR is very important factor alongside 200+ others used by Google in ranking websites.
The main advantage of high PR is to give link juice to other webpage or sell link spot to other people.
That's why it's not that easy to get that. If that will be easy then there won't be much value. You have to spend lots of time, knowledge or money to get that.