Any question answered...

They're different things aren't they. Lines of longitude always circle the entire globe and always pass through the poles. Lines of latitude are always parallel to each other and so never meet and change in size.

Here's one of the worst diagrams I've ever seen (note latitude lines never meet):

If you're asking why we define positions using one converging coordinate and one non-converging coordinate then it's more complicated but basically it's mathematically nice/convenient especially for calculating distances on the surface. It's effectively the spherical coordinate system. We could use Cartesian (x, y, z) coordinates but they're horrible for spheres (great on a flat plane though). We could use cylindrical coordinates (r, θ, z) and that would be okay but the z is less convenient than a latitude (great if you live on a disc - we use them for galactic coordinates).