It is to create leverage to force congress into voting for the wall. This is why Trump continually blames Democrats for the detentions. If it really were in the best interest of the detained children, Trump wouldn't hesitate to take credit. He would never pass up the opportunity to look like a hero. He's blaming Democrats because he knows this is fucked up. He's hoping he can get the majority of the country to blame Democrats and anti-Trump Republicans, and put pressure on their representatives to vote for a wall, but so far just his usual target market of desperate right wing suckers are falling for it.
So basically you just made the case that Trump is trying to fix the immigration problem and the democrats want the status quo for political purpose.
