Drilling gets you resources used to buy various things from the shop, a successful dig to the end of each tile allows you to place a village which gives you increased resources per day. To get the different characters you need to visit different biomes the first time you visit a terrain you can get a new girl with the exception of 3, two of whom are discovered while drilling, you fight one and drill out the other and the last has a sort of little quest to it which you get after going to a specific tile. As of now its more of a sandbox mess about and lewd and isn't particularly balanced in any way but I'm pretty sure that I have read that resource drops etc will be rebalanced in the distant future after more content and stuff is added to the game.