Developers testing their own code is good engineering. It is not automatically a complete test strategy.
A developer is usually very good at proving that the thing they built behaves as designed. A tester is useful when you also need somebody to challenge whether the design, assumptions, integrations and user behaviour make sense outside that implementation.
This is exactly where an independent testing view can help: not replacing competent developers, but finding the gaps their own knowledge naturally creates.
Different questions
Developers often think in components and expected behaviour. Testers should think across journeys, dependencies, failure, data, permissions, odd sequencing and the consequences of the system being wrong.
That does not mean testers possess mystical defect-finding powers. Plenty of testers can spend a week converting requirements into test cases, execute them exactly as written and discover nothing a developer could not have found before breakfast.
The value is in independent challenge, not in the job title.
When a separate tester adds little
For a simple low-risk product with strong engineering practices, good automated coverage, sensible peer review and developers who genuinely test beyond their own tickets, a dedicated tester may add less than you think.
For complicated workflows, regulated or high-consequence systems, lots of integration, messy data, operational dependencies or teams that routinely miss cross-cutting problems, independent testing becomes much more valuable.
Look at the gaps between components
One reason separate testers still add value in strong engineering teams is that failures often live between ownership boundaries. Service A is correct. Service B is correct. The message between them is delayed and the customer gets charged twice. Every component test passes and the end-to-end outcome is still wrong. It is one reason green automated tests can coexist with broken software.
Independent testers are also useful when product behaviour is ambiguous. A developer can perfectly implement a requirement that was a bad idea. Testing should be able to challenge whether the product makes sense, not merely confirm that code matches the ticket.
If you do keep dedicated testers, give them room to think. A tester reduced to executing pre-written acceptance criteria is an expensive human checkbox. The value comes from investigation, judgement and challenge.
If you are trying to decide whether to hire testers, improve developer testing or change the whole approach, get onto us. We can look at how quality is actually being managed and tell you where an independent test function would add value – and where it would just create another meeting.