Out of laziness sometimes I re-heat the water in kettle we use to make formula milk for our daughter, but the health visitor said it's a bad practice to re-boil water over and over. Any specific reason for that?
Water isn't pure H2O; there are all kinds of dissolved substances in it: minerals, chemicals, etc. This is why scientists use only distilled water in experiments (often twice-distilled).
Some impurities will boil off (some volatile organic compounds, for instance) but some will remain behind. With each boil, you lose some of the water to steam, leaving a more concentrated solution of those contaminants which do not boil off.
For example, add a teaspoon of salt to two cups of water; boil away one cup of water, and you'll be left with water that's almost twice as salty as you started with. (Some small amount of salt may be splashed out while boiling.)
This is why you should always start out with fresh water; otherwise you're feeding your daughter water which has more contaminants than fresh water. If you want to do that for yourself, that's your choice, but your baby deserves better.