All men can see the tactics whereby I conquer, but what none
can see is the strategy out of which victory is evolved."

- Sun Tzu

Tactics will increase the amount of damage you will inflict to your opponent with any melee combat skill (Swordsmanship, Fencing, Macing and Wrestling) and one range combat skill (Archery). Evaluate Intelligence plays the same role in Magery. At GM level, you will do approximately 150% damage with your combat skills.  To explain better, here's the calculation example provided by Stratics:

Let's say for instance that you inflict 5 damage with any given weapon at 0 tactics skills. At GM level, you will do approximately 12.5 damage with that same weapon. For every 10 tactics skill points lower, take away 10% damage (ie: at 90 tactics, you will do 140% damage).

You stand a chance to gain tactics everytime you engage in melee combat or use your bow. So basically, you could just fight your way through GM, never really focusing on tactics, yet still gaining.  You will be GM tactics long before you GM your main combat skill.