This is a great 'low hanging fruit' improvement. It's been done in DF Hack before (not available in the 50x release), so it is possible with the current game, and should be simpler to implement at the code level than through hacking. It's also fixes a frustrating experience for new players who try to design their forts just to told they can't.
Two examples of the benefit:
A major benefit of this is it works extremely well with Word Orders. For example if you can dig 30 bedrooms, then design those rooms however you'd like, doors/chests/beds/statues/armor stands/cabinets/etc exactly how you'd like it. Then set a work order like "IF AVAILABLE XXX IS > 5 MAKE 5". From here you are basically done, your dwarves will continue to make as many of the items as they need to and continuously take those items to where they are needed. The best part is when you get another giant migrant wave and you just need to dig more rooms and plan more furniture, then the automation starts up again.
You want to build a wall around your new fort. When you first embark you can plan a large square wall around your fort and set a bunch of trees to be chopped. Then as someone is cutting tree after tree other people are grabbing wood and making defenses. Currently you have to keep building in small chunks until your done, it's a lot of tedious work.
dfhack also featured Item filtering as an option with this. Allowing you to plan a gold chest, or alder wood wall (exactly like with work orders) if that's how you want to do it.
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https://docs.dfhack.org/en/stable/docs/tools/buildingplan.html?highlight=planning%20mode#item-filteringBefore someone says it's not needed because we have dfhack- I disagree that having a hack is a valid reason for not making basic UI improvements the base game. It opens up too many possible problems and it makes a game dependent on volunteers continuing to volunteer their time forever to improve a game with an infinite dev cycle. If dfhack can do this then a native solution would perform better and not need to be updated by fans for every new release version.