The question arrives as a binary: do we take the ready-made system, or have something written for us? Framed that way it becomes a question about price, and ready-made always wins, because the cost of the thing you did not build stays invisible until you are living inside it.
Buy where your process is ordinary and the product is mature. Accounting, email, file storage, payroll: there is nothing to be gained from a bespoke version of a solved problem, and a great deal to lose in maintaining one.
Build where the process is the thing you compete on. If your dispatch logic, your pricing rules or the way you handle a return is a reason customers stay, that logic deserves software shaped around it rather than a text field labelled notes.
Most businesses sit between the two, which is why we start with configuration. Odoo out of the box has an opinion about how a purchase becomes a bill. If yours is close to it, settings are enough and the upgrade path stays clean. Where the business genuinely differs we write a module for that part and only that part, because every line of custom code is a line somebody maintains through the next version.
The trap is the option nobody names: buying something that almost fits. The gap fills with spreadsheets and habits, the workarounds harden into the process, and two years later the company runs on a system it pays for and a shadow system it does not. We would rather talk you out of a build than sell you one.