I think the larger your tractor the more you will use a bucket level indicator. I too can see my bucket from all of my tractors, however, it is not always easy to perceive if the bucket is level even seeing it. The indicator takes the guesswork out of it. I have one tractor that does not have it and I have used it for a couple years and still have a hard time knowing when the bucket is level. If I keep it, I will be buying an indicator to put on it.
As others have said, you have to find what level is on your tractor and make a mental note or tape or paint etc. so you know where you are at. After getting used to the indicator, I can't imagine not using one.