We have exactly one tank manufacturing plant. There is no commercial counterpart. Unless we want to import tanks from another country, we have to make them there.
If we shut it down, then it would have to be completely started back up again. The Congress has correctly decided that the costs of buying a few tanks now to keep it open would be lower than the costs of shutting it down, mothballing it, and then reopening it.
The tanks will eventually be sold as the Army gets a new tank. South Korea retired their last M-47 in 2007. M-48s are still serving in Israel, Lebanon, Morroco, Thailand, and Republic of China, M-60s are still serving in many countries.
Keeping the line open to refurbish the M1 tanks that have been worn out by use in Iraq and need rebuilding. Back during the Cold War, the Army had a major vehicle rebuilding facility in Germany. But that was closed down a decade ago. There is also a vehicle rebuild facility at Anniston Army Depot, AL. And it is cheaper to rebuild tanks than to build them from nothing.