Originally "by writing" meant to write your application on paper, and then send it by mail or, rarely, deliver it yourself. Nowadays it can also mean printing out your application on paper, although there is no writing involved. apply in writing ... to (email address)

What is application in writing?

n. 1 the act of applying to a particular purpose or use. 2 relevance or value.

What is an application in grammar?

plural applications. Learner's definition of APPLICATION. 1. a : a formal and usually written request for something (such as a job, admission to a school, a loan, etc.)