Sun-synchronous orbits are possible due to perturbations caused by the Earth’s shape. These days some perturbations are used to advantage.

The orbital mechanics class that I took three decades ago had discussed these orbits briefly, suggesting the advantage was that a satellite could remain in constant sunshine, so the battery could be small. However, I took that class way back when there were few commercial space companies and none were yet allowed to take/publish pictures of militarily sensitive areas. These days, sun-synchronous orbits are popular for observation satellites, because the shadows are the same with each pass of the same territory, but the batteries have to account for the time that the satellite is in Earth’s shadow.

1. Inclination. If you fly due east from your launch site, you end up in an orbit that matches the lattitude of the launch site in inclination. If you want a higher inclination, that’s easily done by just pointing the rocket a bit north and spending a little extra dV. If you want a lower inclination, you generally have to perform a “dog leg” where the rocket flies south until it’s at the correct latitude, then turns due east (or a bit north of due east until it’s cancelled the southerly velocity). If the starting latitude is sufficiently far off from the desired inclination (eg, trying for equatorial and launching from 28 degrees at Cape Canaveral), such a dramatic dog leg basically isn’t possible, and you need to correct inclination after achieving orbit. Of course, inclination changes are expensive, so you don’t want to do any more of that than you have to, so if you’re trying to reach the moon, the planets, geostationary orbit, or other low-inclination targets, you want to launch as close the equator as you can get.

2. Earth’s rotation. If you’re launching due east, from the equator, you get something like 1,600km/h of free velocity. But if you’re going for a polar orbit, or a sun-synchronous orbit which are often slightly retrograde, then that 1,600km/h is actually in the wrong direction and you have to pay to get rid of it. Launching from a higher latitude, that starting surface velocity is lower, so there’s less to cancel.

John: A high latitude launch site is not necessarily a disadvantage. As I understand it, it is actually a better location for putting satellites into polar orbits. This is why there are a bunch of spaceports proposed around Scandinavia and the UK.