What do Concessions of Land do?

[Ozymandias]

Are they just there to make the citizens happy or do they make money?

[Teleklos Archelaou]

They are a step in the process of having largescale and privatized agricultural holdings for many factions, but for pontos they are a step in having largescale temple farming concerns. Read the descriptions for the nex steps when you upgrade your city again (one with a "grant" or concessio or whatever it is called) - they vary by faction but you'll get more info that way.

[antisocialmunky]

You really should consider buying them as the Romans. *hint hint*

[Omanes Alexandrapolites]

You really should consider buying them as the Romans. *hint hint*

Hi Ozymandias,
antisocial monkey is right - for the Romans (spoilers start here)...

Spoiler Alert, click show to read: 

...six Latfundia, an upgrade of land grants, is part of the trigger for the great Marian Reforms. The other triggers that go along with this one are to be at least year 172BC, hold at least 45 settlements, fight more than 250 battles and have a character that is:

Intelligent/Charismatic/Vigorous
Popularis
Consul or Ex-Consul
Has Influence and Command above two.

Alternatively you can just conquer ninety settlements.

Hope this helps you, cheers!

[Tellos Athenaios]

They are there to make you happy too:
first they have public order effect
their upgrades have economics effects
and their upgrades allow for the recruitment of certain elites (cavalry if I'm not mistaken)

[Rilder]

They are a step in the process of having largescale and privatized agricultural holdings for many factions, but for pontos they are a step in having largescale temple farming concerns. Read the descriptions for the nex steps when you upgrade your city again (one with a "grant" or concessio or whatever it is called) - they vary by faction but you'll get more info that way.

Umm they don't have any descriptions, at least the roman one.