Hollywood always uses the same reasons but they make no since.
Harvest Natural Resources - Almost nothing on earth is unique. Water is like the most basic and abundant compound in the universe. There is no minerals that you could not find in more abundance in single asteroid then the entirety of earth in our own solar system. The only thing that is of any actual value to harvest is DNA and you can get that with a cotton swab. They could use nano machines to collect sample of all DNA on earth and we would never know.
Enslave us - What the hell for? We have trouble getting to our own moon but even most of our big company are replacing humans with machines. Any civilization advanced enough to have interstellar travel would have moved past having any sort of biological workforce. Humans are weak as hell. Why would you use a soft, fleshy being that could die easily when you have a robot workforce that never tires, needs to eat or sleep, and could do 100x the work in 1/100th the time it would take a human to do?
Need us for food - We can grow meat in labs, A civilization with FTL travel would have solved food problems centuries ago. They would have 3d printed super-food or something.
They want our planet - They would most likely have the capabilities to terraform any planet they choose and then clone what ever animals and plants they want. They would have to be complete dicks to choose a already inhabited world to set take. Hell they probably have nano bots that could go to an uninhabited solar system harvest all the planets to create there own artificial rouge planet to use. Why buy a used house when you can build and design your own mansion.
The whole point is that any civilization that has such advanced technology like FTL travel would have solved any and all problems with physical needs that they would have had before then. Hell, they could have ditched biological bodies altogether and uploaded there minds into a sun encompassing super computer where they have made a version of reality that is to them what heaven would be to us.