Reason 2: Better understanding of how/what to test.
It is best to involve testers early, because the initial discussions will help the QA team better understand what needs to be tested, how that testing should be conducted, what systems may be impacted and what exactly is testable.
Reason 3: Optimized resource allocation.
When the QA team understands earlier what testing will encompass, they will be better able to allocate and plan which personnel/tools will be needed towards the project.
Reason 4: More proactive thinking.
QA analysts thinking towards their testing often have some insight that developers do not have and therefore can give preemptive guidance to developers. This occurs often as QA teams may have worked on similar projects and are aware of the pitfalls that may come with it.
Reason 5: Catching issues early.
If the QA and development phase run concurrently, defects in code are caught early. Rather than defective code being deeply embedded in a complete code, time and effort will be saved by beginning to fix defects earlier in the development phase.
Involving QA early in the requirements will optimize project performance and is beneficial to all invested parties.