I use "I think" it to distinguish widely agreed upon facts from my own conclusions.
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.
Going by majority vote, intelligent design is a fact.