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. Add standard solution past the equivalence point to see where the indicator actually changes, then record three titres 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). Anhydrous sodium carbonate (Na₂CO₃) is a classic example: once dried in an oven and cooled in a desiccator it is stable and can be weighed 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.