In contact studies you can identify which structure from a language does or does not influence the other language in use in the same country. I am actually starting to develop something around Andean Spanish for studying Aymara and Quechua influence.
Generally speaking, in linguistic research in general and in contrastive linguistic research in particular, the use of parallel corpora is indispensable. Over the past few decades, there has been a significant use of computer corpora in linguistic research addressing various linguistic domains such as discourse analysis , socio pragmatic contrastive analysis and so on. The reason is that parallel corpora are a valuable source of data which have proliferated the bulk of insightful research in contrastive linguistics. As such, we can say that parallel corpora can serve a number of purposes in linguistic and humanistic studies:
a) They increase the level of rigor in comparative analysis of the two targeted languages.
b) They provide us with an in-depth understanding of micro/ macro linguistic objectives which serve as the tertium comprationis in contrastive research.
c) They can shed light on the delicate structural and cultural differences between languages under investigation.
d) They can serve a wide range of pedagogical and professional objectives such as lexicography, researching nature of language teaching and learning as well as translation.