Parks: do they do anything?
I'm always building parks in every one of my games, but I have no clue why. They don't seem to have that big of an effect on air quality, and they don't have the nifty arrows to show what buildings they effect, so what do they actually do? How much do they effect the air quality, and how much/how far away, do they effect entertainment? There isn't a whole lot of feedback for this in-game, so are they actually worth buildings?
Put yourself into the colonists' shoes: You live on an alien world where the air is thin, has very low oxygen content, and is probably very cold outside, even in the jungle areas. The colony is relatively new, and the only places you can go are your apartment and work. You can't go outside unless you have a heated environmental suit on. Chances are, you will get bored very quickly. Now if, say, the governor was generous enough to build a nice, heated, pressurized park filled with plants, sculptures, and walkways, you would enjoy yourself much more and have some place to go while waiting for the shopping centers, VR centers, and bars to be built. Plus, it would be right next door to your apartment building (at least I hope it would be).
While parks do improve air quality a little bit, they aren't as good as fans and filters. The whole idea is helping the colonists with their psychological health, which is a factor in the game. Plus, these parks are a nice place to retreat to compared to the harsh environment outdoors. I like that they only need 12 nanites to build.
Now parks are not the end-all for helping colonists be happy and healthy. They're just a starting point. So while I recommend a park for each habitat and apartment, it is recommended that you eventually add buildings like hospitals, shopping enters, VR centers, and bars later on.
Usually when I build a habitat or apartment tower, I will surround it with a ring of walkway, so then there's room to place a park and the other fun stuff, in addition to having room to branch out the walkways to various job centers. One of the first items I place by the habitat or apartment is the park. It's not necessary to build parks elsewhere, and you don't want to use up good farmland unless you have to, so that's another reason to keep the parks near the living areas.