The Liberal's purpose of borders is to create a more exploitable underclass of labourers. People who live outside of the borders of Imperialist Nations are treated as subhumans, to be enslaved and looted. Migrants who enter the borders of Imperialist Nations are subjected to extreme legal scrutiny. Migrant workers who speak out against dangerous or unfair working conditions are subjected to threats of deportation or imprisonment.
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If you want what a liberal would say in defense of having borders. They usually invent a scary enemy that has to be kept out of the country. Either communists or criminals. People who "hate freedom and democracy". This is just fearmongering.
In rare circumstances, there will be a Liberal that says like the purpose of borders is to protect the culture of a Nation. That like traditions are sacred and need to be preserved or something. This comes off as like segregationist to me.
You can see the purpose of borders by looking at the history of border enforcement and history of migration. The concept of borders and nations has changed over time, much of it is relatively new. ICE wasn't created until GW Bush.