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1. — And is used between repeated words to show continuation or emphasis.
When the children saw the beautiful Christmas tree they looked and looked.
Old Mr. Bryan has known Grandfather for years and years, since they were boys.
Billy dived to the bottom of the lake again and again, looking for the lost watch.
Everyone wished the speaker would stop, but he talked on and on.
Compare: THROUGH AND THROUGH.
2. — When "and" is used between words with opposite meaning, it often emphasizes how much you mean.
Mr. Jones worked early and late to earn enough to live.
The parents hunted high and low for the lost child.
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Source: A Dictionary of American Idioms