Here's why it might be used:
* Misspelling of "the": This is the most common reason. "Da" might be used by someone who is not fluent in English or who is texting/writing informally.
* Slang/Regional Dialect: In some regional dialects or slang, "da" could be used as a shortened form of "the," similar to how "y'all" is used instead of "you all." However, it's not widespread and considered informal.
If you encounter "da" in a context, it's best to consider the surrounding words and the speaker/writer's background to understand the intended meaning.