Without recourse to introspective judgements, how can ungrammatical utterances be distinguished from ones that simply haven't occurred yet? If our finite corpus does not contain the sentence:
*He shines Tony books
how do we conclude that it is ungrammatical? Indeed, there may be persuasive evidence in the corpus to suggest that it is grammatical if we see sentences such as:
He gives Tony books
He lends Tony books
He owes Tony books
Introspection seems a useful and good tool for cases such as this. But early corpus linguistics denied its use.
Also, ambiguous structures can only be identified and resolved with some degree of introspective judgement. An observation of physical form only seems inadequate. Consider the sentences:
Tony and Fido sat down - he read a book of recipes.
Tony and Fido sat down - he ate a can of dog food.
It is only with introspection that this pair of ambiguous sentences can be resolved e.g. we know that Fido is the name of a dog and it was therefore Fido who ate the dog food, and Tony who read the book.