You have to set the acreage. You don't have to set the acreage for every new batch of workers you get. They will be automatically distributed based on your setting.
It gives you no benefit, to constantly save it.
There is a reason to save it though, even with same settings as doing so will give you the picture in how your workers will be distributed. Then, you can change acres to see how the game distributes workers. These are all legit moves.
Here is an example to understand:
------------------------------------------
A fleet brings in 1200 workers. You have a total of 10,000 workers now.
If you go to infrastructure right now, you will see 8,800 allocated workers.
The 1,200 that just arrived, will not show up there.
However, if you click on manage acres and save, then they will appear.
If you do this to see how they will distribute -> Then it is legit.
If you do this thinking that you need to do it to update- > Wrong. The game will update them on its own and you gain no benefit in doing this manually.