In volumetric analysis, the entire calculation chain collapses if your primary standard isn't absolutely pure and stable — because every concentration you calculate flows from that one reference point. Watch how titre volume changes as you add standard solution, and see why concordance demands titres within 0.10 mL.
A primary standard must be pure (no impurities skewing your molar mass), stable (won't decompose or absorb water from air), and have a known, high molar mass (so weighing errors become negligible). Sodium carbonate (Na₂CO₃) is a classic example: it's stable, doesn't absorb moisture once dried, and you can weigh it accurately to prepare a solution of exact concentration. That exact concentration becomes your reference for every subsequent calculation — if it's wrong, every answer downstream is wrong. This is why glassware must be spotless and why you never estimate the standard's mass.
The equivalence point is where stoichiometry says you're done; the end point is where your indicator actually changes colour — you choose an indicator to make these as close as possible, and you only average concordant titres within 0.10 mL.