A centered column layout keeps the main content in a narrow area in the middle of a wide page. It is like reading a sheet of paper on a broad desk rather than following a sentence that stretches from wall to wall.
Long lines can be tiring to track, so articles and guides often use a limited text width. Extra space on either side keeps the page calm and easy to scan.
On a phone, the column should use most of the available width while leaving a little breathing room at the edges. Centered does not mean every paragraph should be center-aligned.