They increase population growth and give a 10% (I think that's the right percentage) economic bonus.

Fertility center is +25% growth whereas recruiting center is 20% growth and 10% economic.  While you can argue that a stock market is superior to a recruiting center, the instances where you'd rather have a fertility center than a recruiting center are few and far between.

But as you can stack fertility centers, rather than the one-per-planet limit of the recruiting center, of course it is technically better.

The other reason people use the recruiting center is because it's so cheap-it only costs 25 production to build.  (Granted, so do fertility clinics, but you don't have access to them as early.)

Right next to the StarPort, there's nothing equal during early turns but *much* faster planet developments.

I find them exceptionally useful if you are trying to win through conquest (especially when coupled with extra population growth from allocatable points) They help the population of frontline planets quickly get to the point were you can take troops from them without making them vulnerable to counter invasion. the planets might not be good at building transports but it lets you take troops from there instead of your built-up worlds, which can make use of econ improvements you've already built, keeping your economy on track.

I agree. On my small maps, Earth is usually where my transports comes from, and with one of those on the planet I can pretty much  get one transport every few turns and rarely have to slow down because of population drops.