I would like to know the significance and the necessity of on-page optimization in SEO.

On page optimization is done on each page of the website which helps your website to rank higher on SERPs. You need to add Meta Data to each page of the website which should be relevant to the webpage this keyword will help users and search engines to find your website. Your content should be optimized with your certain keywords which should be properly managed with the header tags like H1, H2. On page optimization helps your website to increase its popularity and gain business.