Y do I need coin to lvl up

So I got my cropland to lvl 15 and it says to lvl up I need 10 coin and basic oasis stuff. Why do we need coin to lvl up that point.

Because the people who make Haypi love the dough $$$$$$$$$$$$$$$$$$

wait till you want to go from 19-20.

in my opinion these prices are too high.... Unless you up the crop per hour from the higher levels

[support1]

You don't have to upgrade resource above lvl 15.

You don't have to upgrade resource above lvl 15.

You dont have to, and the output above level 15 is not very significent