You may not have a house plot to play around with, or are playing around with ideas for someone else, may not be able to customize your own house because it will mess up deco etc. There are lots of reasons to use the program. It also has access to tiles that you don't have ingame so it's fun to play around with that way.
This is the main reason it is around.
To make a hosue of your dreams offline. So you will not mess up your real house.
Once you have the house the way you want it. Save it out as an image file JPG, BMP.
If you decide to make the house. Just bring up your image and start making your real house.
Less time building your house in-game. And more time gaming..
If you choose not to use it. That is your choice.
It is freeware after all....