Link building is a practice used by many businesses and individuals to boost your organic rank. This is a grey area. Using a PBN to mass-produce links can get your website penalized by Google if it's gregarious enough. But more often than not, Google will ignore those links because it's rather good at discovering them, meaning you get zero benefits from it. A better way of looking at link building is to create content people will want to link to naturally, not content you have to pay them to look at.