The reasoning is simple. Every system you build is a system you have to maintain. It needs documentation, updates, bug fixes, and someone to own it when things go wrong. If someone else has already built and maintained that system, and you can use it for a reasonable cost, you should probably use it.
Payments: use Stripe. Email delivery: use Postmark or Resend. File storage: use S3 or R2. Auth: use a provider. These are commodities. The implementation details are boring. What matters is that they work reliably and that you don't have to think about them.
The core vs commodity question
The question that shifts the answer is whether the thing you're deciding about is core to what makes your product valuable, or a commodity that anyone could use.
If you're building a fintech product and your novel value is how you model financial risk, you might need to own parts of the calculation logic in ways that an off-the-shelf tool doesn't support. If you're building a logistics platform and your value is route optimization, you probably don't need to own the file storage or the email delivery.
The practical test: would a competitor who used the off-the-shelf version of this be at a meaningful disadvantage against you? If the answer is no, that's a commodity. If yes, that might be worth building.
Total cost of ownership
Buying is not free. A SaaS product costs a monthly fee, which scales with usage. It also costs integration time upfront, time to deal with API changes downstream, and the cognitive overhead of depending on someone else's uptime and pricing decisions.
Building is also not just the initial development cost. It's every bug fix, every security patch, every upgrade, every hour of an engineer's attention for the lifetime of the product. That cost is often invisible because it gets absorbed into the general maintenance budget rather than appearing as a line item.
When you're comparing build vs buy, you need to compare the full lifetime cost of both, not just the upfront cost of building or the monthly fee of buying.
When to switch from buy to build
The clearest signal is when the service starts limiting your product decisions. You want to do something that the provider doesn't support, and the workaround is either ugly or not possible. At that point, you've outgrown the product. The build cost is now justified because the alternative is a product that can't do what it needs to do.
The other signal is cost. If you're paying $50,000 a year for a service that would cost $20,000 to build once and a few thousand a year to maintain, and you're confident the service is stable enough to invest in replacing, the numbers make sense. Most projects don't reach this point early enough for it to be the right call from day one.
The timing question
You can buy now and build later when you have a real reason. The reverse is harder. Building something early means you're maintaining it before you even know if the product works. Use the off-the-shelf option while you're finding out whether the product is worth continuing to build. Switch when you have a specific, concrete problem with the thing you're replacing.