The Windows API follows no standard, it is itself a de facto standard, and deviations from that standard, even small ones, often cause applications to crash or misbehave in some way.
The question becomes, "How do we ensure compliance with that standard?". The answer is, "By using the API documentation available to us and backing that up with conformance tests." Furthermore, a conformance test suite is the most accurate (if not necessarily the most complete) form of API documentation and can be used to supplement the Windows API documentation.
Writing a conformance test suite for more than 10000 APIs is no small undertaking. Fortunately it can prove very useful to the development of Wine way before it is complete.
The conformance test suite must run on Windows. This is necessary to provide a reasonable way to verify its accuracy. Furthermore the tests must pass successfully on all Windows platforms (tests not relevant to a given platform should be skipped).
A consequence of this is that the test suite will provide a great way to detect variations in the API between different Windows versions. For instance, this can provide insights into the differences between the, often undocumented, Win9x and NT Windows families.
However, one must remember that the goal of Wine is to run Windows applications on Linux, not to be a clone of any specific Windows version. So such variations must only be tested for when relevant to that goal.
Writing conformance tests is also an easy way to discover bugs in Wine. Of course, before fixing the bugs discovered in this way, one must first make sure that the new tests do pass successfully on at least the Wine Test Bot base VMs.
Bugs discovered this way should also be easier to fix. Unlike some mysterious application crashes, when a conformance test fails, the expected behavior and APIs tested for are known thus greatly simplifying the diagnosis.
To detect regressions. Simply running the test suite regularly in Wine turns it into a great tool to detect regressions. When a test fails, one immediately knows what was the expected behavior and which APIs are involved. Thus regressions caught this way should be detected earlier, because it is easy to run all tests on a regular basis, and be easier to fix because of the reduced diagnosis work.
Tests written in advance of the Wine development (possibly even by non Wine developers) can also simplify the work of the future implementer by making it easier for him to check the correctness of his code.
Conformance tests will also come in handy when testing Wine on new (or not as widely used) architectures such as FreeBSD, Solaris x86 or even non-x86 systems. Even when the port does not involve any significant change in the thread management, exception handling or other low-level aspects of Wine, new architectures can expose subtle bugs that can be hard to diagnose when debugging regular (complex) applications.