Once a home or building is generating repeat leaks in unrelated locations, the supply lines themselves, not any single fitting, are usually the actual problem. Aging galvanized steel, and in some cases early copper, corrode from the inside at a similar rate throughout the system, so one leak today often means another nearby within months.
Repiping replaces the supply lines with modern copper or PEX, section by section or as a full whole-home job depending on scope. PEX in particular allows a plumber to route new lines through existing walls with fewer openings than a rigid-pipe replacement would require.
Every new line is pressure tested before any wall or ceiling access point gets closed back up, so a hidden fitting is never left behind fresh drywall as an open question for later, since every joint gets checked while the area is still open.