I imagine a satellite meant for reaching a geostationary orbit will benefit most from it, but being near the Equator is surely an advantage to any space launch (right?).
However, what's the impact of each degree south or north away from the Equator on the launch? At what point would a launch become unfeasible?
Obviously, there are other factors. Only 13 countries are crossed directly by the line: Ecuador, Colombia, Brazil, Sao Tome & Principe, Gabon, Republic of the Congo, Democratic Republic of the Congo, Uganda, Kenya, Somalia, Maldives, Indonesia and Kiribati. Most of these are surely not an option. So, I get that any space port would strive to approach the Equator, but might not get too close.