When I'm building, sometimes I idealize the building having two floors, but when I do the interiors, I realize that everything necessary fit in the 1st floor, so there's a doubt: leave the 2nd floor empty or fill it even if it will be redundant/exaggerated? Deleting the 2nd floor is out of question, as the building the building exterior was already based on it. Do you ever experienced?

I'm prone to leave the floor empty, as a decorative floor and sometimes I try to create some excuse for them being like (i.e. the empty second floor in the Bistro is used for parties/events) to avoid a obsession with the "lack of logic" from the emptiness.

I like to use it as a storage area of sorts, or repurpose it into an extra office or bedroom. It just makes sense to me!

Generally, I'd leave it there, usually because I'm more concerned with the external appearance than the internal one. With a residential lot, it's usually a good idea anyway since families grow.

On a residential lot, you can never have too many floors. Eventually someone's going to join the family and they're not going to accept the living room couch as a bed (unless they live in a cramped apartment, which is an interesting challenge for me, since they're always tired). On a community lot, I generally leave it empty and inaccessible, or simply remove some of the floor tiles to create a mezzanine. I can also opt to remove all of the floor tiles but leave the roof intact to create high-ceiling rooms. If it's an owned business, I like to build an office in there for the business owner to veg out in and let his hapless slaves do all the work employees to have a place to rest in, away from the prying eyes of customers.

I've never actually had that problem, but if I did and it looked better with the second than yes I would leave it.

I did have too many bedrooms in my recent beach house that I uploaded so I turned one into a gym, there is always something that you can do with extra space, and once grand kids come along you may find you need every bit of the original.

I will find a use for that floor

I would turn it into a helipad for private helicopter or turn it into bbq area and other outdoor-sy fun stuff to do.

That is, If you don't have walls and roof/ceiling above it.

You can also turn it into a business, by adding stairs from the outside (so that customers into your house and do unrelated/inappriopate stuff.

I'd also probably would leave it as a torture champer for towny sims or even playables.

Now... I never build more than I need as I build room per room when I build. I know exactly how a room will look even before I have begun making the house. I can visually see the house as what it would be in real life and then make it into sims. I have a photographic memory so many of my houses are inspired from in real life places xD

I'll fill it when I feel like filling it. Some of my houses start bigger than necessary as there is an ordinance in SimHampton that houses cannot receive extensions once built and externally decorated, except if a new household moves in (then they have one Sim day's grace to make extensions, provided.they are sympathetic to the existing house design).

I usually ended up with the opposite problem, I'd have an expanding family trying to squeeze into a one-bedroom house, so I'd add extra rooms to the sides and back and eventually add a second floor. I could just move them into a larger house, but many times the family grew extremely fast so they'd have the money for the larger house but no furnishings.

I just did that. I have one unused floor. I used the mansard roof pieces from M&G, so this is why.
Since my building will be turned into an apartment, I will leave it be. There is no stairs that leads there anyway. If I decide to use that space, it will make the apartment cost more.
There is only one unit, but it is on 3 storey. The fourth story would be the empty space..obviously, the roof-room.
Maybe only if I use the building as a single house, then I might end up using that space.

The story of the apartment reminds me of the building in the Downtown Neighborhood with all the extra (empty) floors. It feels as if it once held something much larger than the retail shop on the bottom.
And the building in Belladonna Cove that has empty floors. Of course I don't have all the EPs so maybe they fill with other things if Apartment Life is installed after all the others.