As an example, Russia's launch sites are all suboptimal. Baikanur is at about the latitude of Quebec (not exactly the equator), the launch path is over land, and it's in Kazakhstan instead of in Russia. They recently opened Vostochny on the Russian East coast which improves two of those points, at the cost of being even further north.

For most things, including servicing the ISS, it's perfectly adequate, and for satellites that go into polar orbits it doesn't matter, but some orbits are hard to reach from those launch sites because of the inclination. In the past Russia has used ESA's launch site in French Guiana for those, but that's not an option for them right now.

Geostrategic considerations might make it impossible to use a foreign launch site.