Why do magical traditions specify a spirit type for each spell category?

Question

In the SR4 rulebooks, magical traditions have listings of which spirits a character of that tradition can summon.

For example:

The Aztec Tradition

Concept: ...

Combat: Guardian

Detection: Fire

Health: Plant

Illusion: Water

Manipulation: Beasts

...

Each of the above equates a spell category with a type of spirit. The spirits don't cast spells (other than the ones with Innate Spell power)... do they? It seems to me like the spirit has whatever powers it has, regardless of what spell category it goes with. What does the spell category mean?

Answer 1

Can't spirit lend dice to spell casters to help them cast spells? I'm sorry if this is wrong in SR4, but in the earlier editions that I actually played, you could summon a spirit, and say the spirit had a force of 4. One of that spirit's services could be to help you cast a spell, adding 4 dice to your pool, but only certain spirits could help with certain spells. Like, fire elementals were useless at helping you cast healing spells. Fire elementals can only help combat spells. In the Aztec Tradition, I guess, water spirits cannot spend a service adding their force in dice to your spell pool when you are casting combat spells. Water spirits can only help with illusions.