[I think] That just makes it sound like you lack confidence.
It's also a way to differentiate objective statements from subjective statements: I think Metallica is a great band.
A true argument can stand on its own.
If I say something potentially provable that I am unsure of but claim is a fact ("Metallica is the best-selling band of all time"), this would be misleading, and I would be taking advantage of my audience.