Visual acuity - "how low can you go" on the eyechart - comes in two flavours: uncorrected visual acuity (UCVA), meaning how you score with your unaided vision, and best corrected visual acuity (BCVA), meaning how you score with the best possible glasses prescription for you.
While improving the UCVA is the goal of laser eye surgery, maintaining the BCVA is a very important safety standard in surgery. Suppose for example, that without glasses today you see 20/400, but with glasses or contacts you can see 20/20. Then suppose you get laser eye surgery, and afterwards WITHOUT glasses you see 20/30, that is, you have a UCVA of 20/30. That is a tremendous improvement. But suppose at the same time, your BCVA after surgery - the best acuity you can achieve with glasses - is also 20/30. That means that even though you are now not dependent on glasses, you cannot see as well as you could prior to surgery. A goal, therefore, of laser eye surgery is to maintain your BCVA.
It is also important to note that BCVA does not measure all aspects of one's vision. That is, a patient who has the same BCVA after surgery as before, may still feel that he does not see as well as before, due to aberrations or other factors which have worsened his vision quality.