For this reason, we recommend testharness.js for OM-related tests, but unless there's a very strong reason to use scripting for a test, layout tests should be done as reftests.
Short answer: partly to help us evaluate coverage, and partly because, like commenting your code, it helps reviewers and maintainers understand what your code is trying to do.
Some test writers find that writing out the assertion helps them think about what, exactly, they're trying to test and how it relates to prose that is in the spec. (And if that prose is not there, it's time to file a spec issue. ;)