This pattern says that although something is not literally true, it is close enough to be considered the same. It often carries a sense of exaggeration, resignation, or strong assertion. You'll hear it in spoken Japanese, but it also appears in writing. For example, after someone works for a company for decades, you might say it's 'as good as being family.' It's a subjective judgment, not a factual comparison. The verb form is typically past tense because you're comparing to a completed action or state. Note that when used with the past tense, it emphasizes that the result is equivalent to having done that action. With nouns and な-adjectives, も同然だ attaches directly to the noun or stem.