The reason why is more up to the individual person. There are a lot of reasons why an elf would mark runes on their bodies; either for greater arcane empowerment, protection, or strength.
Here are some wowpedia sources on runic magic, and runemasters. Hope they help to clarify your question and/or concept!
“Those who seek to understand and master runic power understand that the convergences of the ley lines are not the only important part of this power — the patterns formed by the lines themselves are important as well. These patterns, which resemble letters in a strange alphabet, are called runes. Each rune is a reflection of a pattern of energy lines somewhere in the world. Each such pattern creates a certain magical effect when energy flows in that specific formation, though in its natural occurrence, this effect is much too broad and scattered to have a meaningful impact.
Those who master runes, then, seek to duplicate those patterns in smaller forms; their runes manifest magical power that is arcane in nature, but purely benign and non-corrupting. They are manifestations of Azeroth’s own unique flows of power, nothing more.”
"Most runemasters are heavily tattooed with runic patterns – not simply the unique ones that form the basis for rune families, but the so-called "simple runes” as well. Simple runes mimic patterns of ley energy that appear, again and again, in the environment. These patterns occur in nature and are responsible for things such as the strength of a given type of stone, the ferocity of a given windstorm and the coolness of a given stream; when a runemaster uses these patterns, he learns to channel his power into them at all times, attaining similar traits.“
this is why dax has tattoos.
In the Scourge when the Lich King says “Go get runes tattooed all over your body“ you were probably walking in that direction already anyway.
Thus Tev has runes all over his body.