If we let our imaginations run, they will become fantasies we believe and in turn exalt themselves above the knowledge of God and His commandments...
Satan plants simple thoughts that are enticing, we run with them. They are founded on lies but because we like them we agree and hold them. These thoughts mixed with our desire and emotions will eventually lead us into acting out and these actions lead us out of God's will and in to sin. Then in the end we feel unfulfilled, empty, and still desiring more.
These imaginations must be identified for what they are in the beginning, lies, shortcuts and counterfeits. Capture them and let Jesus shine truth, He will expose the lies. Live in His truth so when these imaginations come they can not stay and eventual exalt themselves above God in His own temple (which is you). This was satans plan from day 1 (Isaiah 14:12-15).