This is so that as many trusts can be created as are necessary so that each of your beneficiaries may in due course have their own trust with themselves as the primary beneficiary and with each trust in identical terms.
So, for example, the initial trust may only be for your spouse. Your children are further potential beneficiaries of that trust and are persons to whom your spouse is permitted to distribute capital or income of the trust (thereby assisting with your spouse’s own tax outcome from year-to-year).
The most likely use of the parallel trust provisions would be after the death of both you and your spouse at which time all the remaining assets could be distributed equally to a separate trust for each of your children with each of those children being the primary beneficiary and trustee of their own trust. In that way they would then have control of their own trust for the benefit of their own families and without needing to consult with their siblings.