A file can contain only digits without describing a number. Badge 0007 is a label; subtracting it from another badge has no meaning. Keep identifiers and postal codes as Text so the leading zeros survive. Hours and scores are measures, so those columns need numeric types before you calculate with them.
The type icon in each header tells you what Power Query currently knows. The ABC 123 icon means Any, which leaves a column without a declared type. Choose a type based on the business meaning, then inspect the values. Converting an identifier to Whole Number and back to Text doesn't restore lost zeros.
A missing reading is also different from a reading of zero. Type changes preserve null values. Don't replace a missing value with zero unless the data owner has said that zero is the correct business meaning.