In China, a nationality (zu in Chinese) is not only tied to an ethnic identity, it also constitutes a political status that implies legal rights for the individual or group concerned.
A person from a minority nationality benefits from certain positive-discrimination measures; for instance, the single-child policy in force is less strictly applied in this case; the person may have preferential access to higher education, etc. Claiming membership of a recognized minority nationality in China also makes it possible to tell who is a subject of the Chinese State and who is not. Political ulterior motives aside, claiming a nationality may avoid confusion and misunderstanding at a practical level. This may explain why the Chinese transcribe the name of a people differently depending on whether it is considered a minority nationality of China or it lives across the border, as is the case of the Evenk of Russia, transcribed Aiwenji, whereas they are the same people as those called Ewenke in China.