The four constraints that justify it
Plus earns its price when a specific limit is costing you money, and you can name the limit. In our experience these are the four that come up.
Checkout logic that has to be yours. If you need custom rules at checkout, such as different rules by customer group or market, the standard plans give you limited room. Third-party upgrade guides like MESA's Shopify vs Plus comparison describe the gap as complete control on Plus versus limited modification on standard plans. If checkout friction is the constraint, price it: a small lift in conversion, multiplied by real order volume, is easy to compute, and it is the number that should decide the upgrade.
Wholesale next to retail. Shopify describes Plus as including tools built for wholesale buyers. If a real share of your revenue is trade accounts with their own pricing and payment terms, running that inside the same store beats running a second one.
More than one storefront. Several brands or markets, each needing its own store, is where Plus starts to be a bargain rather than a cost.
Operational scale. Shopify states that Plus handles more than 10,000 checkouts a minute with a 99.99% average uptime. Those are Shopify's claims, and they matter mainly if you run flash sales or heavy launches, not for a store with steady traffic.
If none of the four describes a problem you have today, the case for Plus is a bet on the future, and a three-year term makes that bet expensive to reverse.