Well there are two things to consider here , on page has alot to do with relevant information , the ease of user interface , and general fixing of errors or bad coding withing the site. You would be surprised at how many people neglect these things and the impact they actually have , i used to do mostly off-page seo , link building, social media spreads ect and yes it gave me some growth in serps. I did on page once and properly and i jumped withing a day or two atleast 8 positions in most of my keywords. That just shows me that those websites that were ahead of me also like myself neglected on page seo as they mostly have greater authority than my own website. So i spend most days tweaking my content , and site structure bit by bit and of course some off page seo too.
One thing id like to say if you have super off page search engine optimisation , but bad on page optimisation there may come a time when you rank well. Though the whole world will know your site but the lack of on page attention will break your brand. Get the on page stuff right first before you show your stuff to the world.