Isn't creating a farm and raiding someones base just the same?

In general yes. Your farm(s) are there for you to raid and take the resources. The advantage to farms is you can control the resources, know what you can take and also make sure there is no defense to kill your troops. Plus no chance of retaliation from an alliance or having the enemy shield up or run away. An easy target, especially if you have a high NAP protected state. You can do both of course.