In what way do caravans improve roads? Quicker travel for all units, just caravans or something else?

I think if your faction's caravan successfully navigates the wild world to an AI's city when he delivers his good he gets 20 pennies and a sense of job satisfaction. 

It's either that or they are seem like they are completely worthless units that take up space on the strategic map and AI's like to cast wither on them.

trade treaties are very useful in the early game, they're a good source of income when you still have low level towns.

Caravans are worthy only if you dedicate your capital to gildar production and if you have 5 or more AIs to trade with (each one gives +10% to the income of the capital), but it will need the Legacy of Serrane trait (bazaar improvement ad invulnerable caravans), a lvl 5 town capital (not conclave/fortress) with 2-3 essences and the propaganda spell, the Treasury Vault world achievement and Produce Wealth on - not a easy task! But then you'll swim in gold and can rush basically every production. Anyway, caravans are way underpowered at the moment.