NC bill that permits leasing of muni fiber raises question of how it reduces infrastructure construction costs

Rural broadband internet bill passes NC House | News & Observer: House Bill 68 is titled the BRIGHT Futures Act, which stands for Broadband, Retail, Internet of Things, Grid Power, Healthcare and Training. Its main goal is expanding access to high-speed, broadband internet in rural areas of the state where it’s often too expensive for private service providers to extend their lines.

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Bill sponsors say cities, towns and counties could make use of what’s known as “dark fiber” – additional capacity in existing infrastructure that government uses to connect traffic lights, schools and public facilities. Leasing capacity to private internet providers could provide revenue to local governments that could support the expansion of
government-owned fiber networks.

The problem as stated here is it's too costly for private telecom providers to extend their infrastructure to rural areas. So how does allowing these providers to lease local government owned fiber address the cost of building the needed infrastructure that connects homes and small businesses? Particularly given the challenging business case to justify building infrastructure to serve these areas?