Why WRG 6th Editon Ancient Wargaming Rules?
Why 6th over 7th? The appeal of 6th is that it is the last (and best) of the 'battlefield casualty' systems before whole-figure/base/unit removal became commonplace. Furthermore, the troop interactions and results are much less abstracted than in later sets, and are for the most part highly realistic (although you will observe the occasional trifling difference of opinion between gentlemen over exactly what constitutes 'realistic').
7th Edition began the change away from individual casualties and detailed troop interaction to abstraction and whole-unit removals (although it took 15 steps of losses/fatigue per figure to get there and your effectiveness declined with each 5 steps). It was faster, and it could handle larger forces, but the 'tabletop clash of arms' element was attenuated: it felt more like a boardgame with miniatures. And the rules had rather more interpretation questions and anomalies than were ever found in 6th (direction changes during march moves being a case in point). So the popularity of 7th waned, although it still retained its adherents, and then came DBM, which took speed and abstraction to the nth degree.
6th edition remains the premier 'realism' set, with a highly sensitive and detailed troop reaction test system and interactions at the unit level resulting in individual casualties which have immediate tactical effects and slow attritional impact on fighting strength and morale. It may take forever, but it is a great deal of fun and usually good history. It is about as close to the real thing as we are likely to get without re-enactment. Therein lies its appeal, for me at any rate. Others will undoubtedly add their own thoughts and reasons.