One of the pitfalls is split incentives. Builders and landlords pay for the insulation/appliances but buyers/renters pay for the utilities.
You can also put it on the outside to give yourself 'triple glazing'. It makes a really big difference to the heating required to stay warm.
Looks a bit tacky though.
It's completely mind boggling that anyone was allowed to build housing with single pane windows in 2003.
If the cost between a 2002 and 2003 home is nothing and they’re basically the same, you might find one is much cheaper to insure... (I don’t recall exact years but I can look it up if people care)