What are runway Aiming Point markers used for?
I'll try to answer just the first portion. The aiming point marking basically serves as a visual aiming point for a landing aircraft.

Image from code7700.com
They are not found in all runways because the requirement is based on the aerodrome code number and they are not required in all cases. Aerodrome Reference Code from ICAO Annex 14 Volume I:

Table from ICAO Annex 14 Vol I
From the Same reference:

5.2.5.1 An aiming point marking shall be provided at each approach end of a paved instrument runway where the code number is 2, 3 or 4.
5.2.5.2 Recommendation.— An aiming point marking should be provided at each approach end of:
a) a paved non-instrument runway where the code number is 3 or 4;
b) a paved instrument runway where the code number is 1;
when additional conspicuity of the aiming point is desirable.