You receive coins on a regular basis from your residential buildings, but once a day there is also a daily bonus that you can pick up in your town hall. In addition, you will receive coins every now and then if you successfully complete quests, bring sectors or provinces into your possession or plunder from the enemy. You need coins to build new buildings, to buy expansions or to recruit military units. Furthermore, you need them to unlock technologies, for scouting provinces on the continent map, to infiltrate before a battle and to buy additional Forge Points. So keep in mind: Coins are important, you can never have enough!