A Function computes the outcome you intended rather than hoping several discounts cooperate.
Functions run server-side at checkout, so a discount cannot be visible in the cart and absent at payment.
They hold through accelerated checkouts, unlike anything implemented by manipulating the cart.
Discount Functions are available beyond Plus, which is a meaningful widening compared with the old Scripts boundary.
Build configuration into the Function from the start, or every future threshold change becomes a deployment.
Shopify's supported mechanism for customising platform behaviour with your own code — discounts, delivery options, payment methods, cart validation.
The full guide has the formats, the deadlines and the worked examples.