Expressing "Both A .. and B"
我是一个语言的从教者，也是一个语言学习的爱好者。


This sentence reminds us of the fact that I'm also a language hobbyist(assuming I'm well known as a 语言的从教者). 


  我既是一个语言的从教者，也是一个语言学习的爱好者。


With 既, it parallels the two roles (语言的从教者 and 语言学习的爱好者), which share the same level of importance.