Because that's what you are asked to enter on the form under Questions 1.16 & 1.17.seagul wrote:Why address should be where the applicant live?
You would need to weigh the probability of the the rules changing before you have moved, if you decide to delay the application by a long period of time. Based on the past, Home office fee increases usually occur in April every year, however legislation changes can come into effect at any time. As seen over the last couple of years, rules changes come into effect at different times throughout the year.
If your home move is not imminent, why not just apply now (if you meet the requirements & have all your documents ready) and then notify the Home office of your address change, as suggested before, once you have moved? That appears to be the most straightforward option.