When would it be advantageous not apply Training Ground's cost reduction?

Question

Training Grounds has an optional cost reduction ability, as indicated by the card-specific ruling:

You may choose not to apply Training Ground’s cost reduction effect. You may also choose to apply only part of it (causing an activated ability of a creature you control to cost just {1} less to activate).

Why is this conditional? Training Grounds never co-existed with Mana Burn. Are there any situations in which it would be advantageous to not apply this ability?

(This is related to Paying more mana for a Flashed creature, but is about why you would not apply the cost reduction rather than whether or not you can.)

Answer 1

The main advantage here is in very niche cases where you have lands or other effects that add more than 1 mana and cards that care about the amount of mana in your mana pool. Glissa Sunseeker, for example, has this ability:

{T}: Destroy target artifact if its converted mana cost is equal to the amount of unspent mana you have.

If you have an activated ability that costs more than 1 mana and you can only activate it a limited number of times, the flexibility of Training Grounds's cost reduction gives you a wider range of amounts that you can end up with in your mana pool, giving you a wider range of targets that you can hit with that ability.

Similarly, Omnath, Locus of Mana gets a power and toughness bonus based on the amount of mana in your mana pool. If you want to dodge Legion's Judgment while keeping it as large as possible, you have the flexibility to do so.

Answer 2

You have a Glissa Sunseeker in play, and your opponent has an artifact with CMC {1}. You have 1 untapped Lotus Vale and no other untapped lands or mana in your mana pool.

You want to activate an ability on a creature that normally costs {3} (like that on Walking Wall), and you also want to use Glissa to destroy your opponent's artifact.

After getting 3 mana from Lotus Vale, you need to spend exactly 2 mana in order to have {1} remaining, so that you can destroy your opponent's artifact. So you choose to only reduce the cost if your ability by {1} instead of reducing it by {2}.