Every PBN owner wants to keep authority of the network as long as possible without dropping metrics and Trust Flow is one of the most trusted metrics representing the value of your domains.
Trust Flow of your PBN almost inevitably will be dropping over time, because your domains will be losing high TF backlinks. And to keep your metrics you need new backlinks from other high Trust Flow resources. You can buy PBN links from other vendors, but that would be quite an expensive way…. Or you can use the method I discuss in this episode.