So as to create a particular location on the grid where they intersect.
Railway lines with sleepers? Lines of latitude crossed by a line of longitude?
Each 'meridian' is a line of constant longitude.
All lines of longitude are equal. The longest line of latitude is the Equator.
Every line of longitude intersects with every line of latitude and vic-versa.
All lines of latitude are parallel with the equator.
The Equator is a longitude line. Latitude lines pass through the equator.
Lines of Latitude encircle the Earth horizontally, and are parallel to the Equator. Lines of Latitude are intercepted by Lines of Longitude. Lines of Longitude run vertically from the North Pole to the South Pole. The Prime Meridian is zero longitude and passes through the Greenwich Observatory, Greenwich, London, England.
Every line of constant latitude intersects every possible longitude, and vice-versa.
The lines of latitude provide vertical (north-south) coordinates on a map or globe. Lines of longitude provide horizontal (east-west) coordinates. The defined geographical point is where the latitude line intersects the longitude line.
The lines of latitude and longitude are important because we need them to describe exactly where a point is on Earth.
The 60th parallel South is a line of latitude crossing all lines of longitude.
The imaginary semi-circular "lines" of constant longitude are "meridians". The imaginary full-circular "lines" of constant latitude are "parallels".