Generally, Americans “celebrating Columbus Day” just means they take the day off. People don’t really celebrate Columbus outside of specific communities which are largely Catholic and/or Italian. Some states don’t even officially recognize Columbus day.
Columbus is often credited with establishing European contact with the Americas as a whole, even though he didn’t discover the region or set foot in what is now the USA.
Columbus Day was largely pushed on Americans by Catholics (and some protestants) who recognize Columbus as a key figure in the worldwide spread of Christianity. He is also celebrated by Italian-Americans of which there are many.