You'd be very surprised. Sometimes timing issues are not even noticeable by the naked ear. You could be off by a tenth of a second and not even notice. So what does it matter then? You might ask. In your 3:30 min song, if you are off a 1/10 of a second, every second, it adds up. If you are jamming live, you and the band won't know. On a recorded song, it will be as clear as day. That's if your a good drummer. Someone far worse won't be off by 1/10 of a second. They will be off by a LOT more.
Get a click track and learn it as a second nature. You will be far more useful to any serious band. It is NOT just something for recording studios. ALL major touring acts will have a live drummer that uses a click. In their in ear monitors they will have the music in one ear and the click in the other. They can change volumes and settings accordingly. A band trying to become pro will never make it without a tight drummer that can play to a click.