What is the benefit of changing Primary Spell Factors?

Question

I've a fair amount of experience with Mage: The Awakening, first edition, but it's been several years, and I'm trying to learn again with the new system from Onyx Path Publishing. In the rules on spellcasting it says that you can alter a spell's Primary Spell Factor by using a reach.

I now understand what changing the spell factor is. It applies in cases where I might want to change the spell's primary factor from potency to duration. My question is: Why would I want to do this?

If I can already alter the range, duration, scale, and potency, why would it ever be worth investing one of my limited reaches to changing which factor is considered primary?

Answer 1

When you cast a spell, the primary spell factor may be increased by a number of steps equal to the caster's highest relevant Arcanum less one for free (Mage: the Awakening, 2nd Edition, page 113). By changing the primary spell factor, a mage may effectively apply this increase to some other spell factor.

An example:

Flux the Acanthus decides to cast Temporal Stutter on herself to avoid a fight. Ordinarily, the primary spell factor of Temporal Stutter is potency. However, since she's casting it on herself, she doesn't need to worry about the spell being withstood, and would prefer the effect lasted for longer. By expending one reach, she changes the primary spell factor to duration. As an Adept of Time, this will mean the duration of the spell is increased by three steps (her Time of 4 less 1) along the duration chart from 1 turn to 5 turns, plus any additional steps she gets by taking casting penalties.

Answer 2

Basically because increasing spell factors costs dice from your spell casting pool, and you might want to avoid that from happening, either because you want to minimise the risk of failure with a small pool, or because you want to maximise the chance that you’ll score an exceptional success with a larger pool.