It signifies that something is so similar to another thing that it can be considered the same, even though technically it might not be.
Here are some examples:
* "After the accident, his career was to all intents and purposes over." (Even though he wasn't officially retired, the accident effectively ended his career.)
* "The new system is to all intents and purposes identical to the old one." (There might be minor differences, but the systems function in the same way.)
* "She had been living on her own to all intents and purposes since she was 16." (Even though her parents still provided some support, she was essentially independent.)
The phrase is often used to emphasize the practical reality of a situation, even if there are technical or legal distinctions.