More context: contract

Contracts need to be balanced (as all things should be). The reward the person gets for the price they paid must be equal in value, most of the power she gains is taxed while under the contract.

Ness doesn't actually need the power from the contract, she saw how she was struggling and put her on the path to being a tantric witch. But she couldn't just give her the abilities as there are rules she must follow.

So she essentially gave her a training contract. All the abilities she gained from the contract were structured in a way that she could learn them. If she continues down the path she will be able to cancel the contract and keep all of the abilities.

Of course she will have to learn some abilities of her own to really thrive, either in the form of spells or a renegotiation of her contract.