cuz batts have charge cycle de ... that is why frequent charge will render the batt unusable when approaching the end of the cycle charge.

Lets say the batt for example has 3000 cycle charge... means each time the batt encounters a charge then it minus 1 cycle charge. So even ur batt is in 100%, and you plug the power cord to ur notebook and that would cost you 1 cycle charge gone. So always advise to remove batt before plug in power.