Why dead legs fail first
Flow is protective: it sweeps water and solids along, renews inhibitors, and starves biofilms. Remove the flow and everything changes — water settles at low points, deposits blanket the bottom of the pipe, oxygen differentials form under them, and microbial communities move in. The result is under-deposit and microbiological attack concentrated exactly where nobody is looking, in the branch that "does nothing."
Managing them
The hierarchy is simple: eliminate dead legs that serve no function (cut and cap at the header); drain and monitor the ones that must remain, with dedicated inspection points placed at their low points and stagnant ends — not on the flowing header nearby; and review after modifications, because every project that reroutes flow manufactures new dead legs out of yesterday's mains. In-service inspection codes expect dead legs to be identified and specifically monitored; an inspection plan without them is monitoring the healthy pipe.