Why five touchdown zone markings for only one aiming point?
ICAO Annex 14 specifies the need for 6 pairs of touchdown zone markings for runways with a landing distance available of 2400 metres or more.



As noted in Annex 14, the number of markings relates to the landing distance available.

Presumably, the rule is there to make it easy for a pilot approaching the runway to make sure the runway is long enough if, for some reason, they are uncertain of the runway length. Similar logic could probably be applied to the rule regarding threshold markings ("piano keys"): What does the number of white bars in runway&#39;s threshold mean?