When proofreaders take out their red pens, their main purpose is to get rid of any errors that may hinder comprehension or sidetrack the reader. One aspect of this task is a certain economy of corrections: a proofreader should spend their time on warranted changes. These changes have to do with grammatical errors and with awkward usage.
The online writing lab at Purdue University provides many resources for learning to identify errors in the mechanics of language. These formal errors cannot be left on a page, because what is left may often not qualify as English language. They are like a sinkhole in the middle of a city and must be dealt with swiftly.
Usage reflects how people connect words with their meanings today. Usages change over time, and ignoring these trends can have catastrophic effects. Take for instance the word “convent,” which used to mean, “a community of religious persons,” but since the 1800’s has been used to describe communities of religious women exclusively. A phrase such as, “brother George joined the convent,” may sound odd in our century, but in older texts and reconstructions, it is perfectly correct.
There are dedicated search engines that pull references from literature and academia to describe exhaustively the usages of specific terms or phrases. However, for a quick check, there is the Google Books Ngram Viewer. It’s a neat tool for finding frequencies of expressions across time, in a large corpus of books which spans several centuries.
Of course, we are always here, should you need any proofreading help.