Why was the Marshall Plan good for the U.S?
Fewer refugees, allies were in place for a follow-up war with the USSR, and trade resumed with Europe a lot sooner.
Before World War II ended, the three main Allied leaders--Roosevelt, Churchill and Stalin--discussed how to divide up postwar Germany. They also divided up the rest of postwar Europe and Asia, and all parties felt a bit shortchanged. There was an unspoken assumption that another war would follow, between America and the Soviet Union; and if it occurred in Europe, both sides wanted allies at the ready.
America dealt with this via the incredibly benevolent-sounding Marshall Plan, which disbursed funds to rebuild a lot of the destroyed cities and infrastructures of Western Europe. The subtext of this unprecedented generosity was, "if the Commies start another war, you guys owe us one." (Not an actual quote, but the subtext was unmistakable.)
The Marshall Plan also stemmed a vast tide of European refugees seeking asylum in America after the war. A massive influx of dispossessed immigrants would have put a big and immediate strain on the US economy that was already struggling to repatriate 10 million + American veterans into civilian life. Cutting a check for Western Europe could just get tacked onto the already-staggering national debt; millions of immigrants would have been more expensive and difficult.
The Marshall Plan also put European countries into a position to resume trade with the US. The lack of trade with European markets hurt many American businesses, who tried to make up the difference with expanded trade with South America, with limited success.