Is insulating a garden house sensible or unnecessary?

A garden shed is not completely airtight, even with a floor. But if you skip the floor and place it directly on the slabs, all the better. Then you have regular air exchange through the gaps between the slabs and certainly no problem with mold. As long as the foundation is sufficiently above ground level and no rainwater runs in, I wouldn’t go so far as to make it completely airtight with a foil.

Is there air exchange through the gaps between the plates? Through the floor? How should I imagine it? I guess without a vapor barrier foil, more moisture comes up from below than goes down through the floor or out. Ventilation is rather provided by the air vents integrated into the roof..... I think there are also air vents in other places, but I’m not sure. If necessary, you can also drill holes yourself or install a ventilation grille. All of that is comparatively easy to do afterwards, but any sealing/changes to the floor are difficult. The house will be more or less at lawn soil level, maybe slightly above.... I actually want it to remain as level with the ground as possible. However, the area will have a slope so that water drains off. Therefore, apart from flooding (which here would have to be 80-100 cm high, since the property slopes and is built up), no water will run in.

I would do without any insulation as well as foil or anything else dense under the panels.

By insulating, you turn the garden house into something like a thermos flask. Once cold is inside, it stays there longer. The same applies to heat. The interior is not supposed to be heated, or did I miss something?

To prevent water from penetrating at floor level, I would lay a row of panels outside with a slight slope away from the garden house.

By the way, depending on the soil conditions, you can also simply lay the paving slabs directly on the smoothed earth. I did this on our garden path decades ago, and the slabs are still lying just as they were at the start.

Is there air exchange through the gaps between the slabs? Through the floor? How am I supposed to imagine that? I assume that without a vapor barrier foil, more moisture comes up from below than goes down through the floor or out.

You have a layer of gravel under the garden house that breaks the capillary action. Air that comes in through any cracks can also escape through such cracks. Don't think of a house, think of a shelter with walls.
Between the slabs you have either grit or sand. That does not seal airtight. And if you already have ventilation slots in the roof, all the better. I see absolutely no need or sense in insulating and sealing a paved surface airtight.

I think the way we did it was just right. No ribbed steel floor, but the paving slabs, all filled with grit. And swept with quartz sand. Ventilation always takes place somehow, no condensation forms. The climate is fine.

I saw no reason to install a vapor barrier.

What I would have done if the house stood in direct sunlight: maybe insulate the roof with Styrofoam, but only because of the heat.

These houses are incomparable in terms of comfort and steel thickness to 199 euro metal sheds from the hardware store.

I think the way we did it was exactly right. No ribbed sheet metal floor, but paving slabs, all filled with grit. And swept with quartz sand. Ventilation always happens somehow, no condensation forms. The climate is fine.

I saw no reason to install a vapor barrier.

What I would have done, if the house stood in full sun: maybe insulate the roof with Styrofoam, but only because of the heat.

These houses are in terms of comfort and steel thickness no comparison to 199 euro sheet metal huts from the hardware store.

Our garden house has 44mm walls. Concrete slab with bitumen membrane against moisture from below and insulated windows. The roof was insulated by the roofer because of heat in summer (sun from above). Otherwise, the insulation without heating brings just 2-3 degrees difference in winter. That means at strong subzero temperatures a small frost guard runs with 400W because of the Mediterranean plants overwintering in the garden house. Last winter at -15 to -18 degrees we had -7 degrees inside the garden house with frost guard and various candles. But everything survived.