Use 'drop by' for unexpected or short-notice visits to someone's place.

Impromptu Visits

'Drop by' has a casual, informal tone, making it suitable for friendly encounters, not business.

Casual Tone

Suggests a short visit, usually without specific plans to do something at the location.

Use 'drop by' when indicating the degree to which something decreases.

Most effective when quantifying change, especially with numbers or percentages.

Unlike the first sense, this use does not relate to physical motion or travel.