I think it can be used for different functions for different types of player.
For example if you havea player who is poor at or has no interest in tactics, it can be used to supply tactics or suggest courses of action if the player feels "stuck"
If you have an average player it can be used to check the workability of strategies or suggest alternate approaches.
If you have a very strategically minded player it can be used to supply a piece of information they couldn't otherwise get or a course of action they can't actually see.
For example seeing a building thats occupied with traitor guardsman, a tactics roll might let them know that the typical size of that unit is 100 men, or their doctorine states to store their fuel and ammo on the left hand side of the bulding on the ground floor.
In many ways its the "Being a Space Marine in a War" skill, if the player is out of his depth or looking for options in a military situation, Tactics is a good skill to roll.