Testing Throughout software lifecycle is necessary in fact, because testing starts should the early stage of the product life cycle. it starts from requirements phase where testers would be understanding and discussing requirements with business analyst.
In Design phase, testers would be creating test scenarios and test scripts.
once the product is deployed, testers would execute the scripts to uncover the defects.
post deployment, they might be supporting the production defects testing. So yes in every phase QA team is involved.
In waterfall model, testing was the last phase in development life cycle but since it was costly to fix the bug or any feature changes at the end of life cycle, new software models were developed.
When the development team gets the Requirement spec, the testing team also gets the same requirement spec. The testing team should go thorough the spec and decide on how the approach should be towards testing the spec, what all features to be testing and how? Is it possible to automate any of the tasks and if yes, then which tool to use and check whether the team requires any training for it.

Even developers do some testing once they have the initial draft of the product, it is called Unit testing.

When you go into Agile models this becomes more intense, as our Anonymous contributor mentions. Developers are empowered to drive the quality of their product, but this empowerment comes together with the responsibility to make that real. In each phase of each iteration in the process.

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