Why do we say "as it were"?

Solution 1:

Interesting thought: that as it were might be an idiom, used to emphasize that something else in the sentence is also an idiom.

Merriam-Webster's online dictionary defines as it were thusly:

as it were :
as if it were so; in a manner of speaking

Wordnik lists these synonyms:

• so to speak
• in a way
• in a manner of speaking

Solution 2:

It's an example of English subjunctive mood (one of the irrealis moods).

This particular example is a set phrase (relic from an older form of the language where it was much more common) where subjunctive needs to be employed.

Solution 3:

It is used, in the words of the Oxford English Dictionary, ‘as a parenthetic phrase . . . to indicate that a word or statement is perhaps not formally exact though practically right’. It’s very old, being first recorded around 1200.