Web 2.0 websites are the websites which are run by user-generated content where they meet and share data and, in the process, generate data which are used to curate their experiences and that forms numerous communities.
The main purpose of link building in Web 2.0 is getting sufficient backlinks from a trusted and big user base community website, which will prove very beneficial as it will be shared among many members and it comes with authenticity.
Getting more backlinks mean more ranking, this is an SEO strategy which is beneficial for the website as it drives large traffic.
If you want to become an expert in SEO, then I would recommend you to take up the SEO course from Intellipaat, moreover I would suggest you watch our YouTube tutorial video on SEO to gain more insights.