Drywall finishing is graded in levels, and the level is a decision about where the work is going, not about how much anyone cares. A level that is right for a storage room would be visibly wrong in a lobby, and a level right for a lobby is money wasted behind a tiled wall.
At the low end, joints are taped and left — fine where the surface will be hidden behind tile or cabinetry, or in spaces nobody looks at closely. In the middle range, joints get two or three coats and are sanded smooth, which is the normal standard for walls that will take a flat or matte paint and are lit from ordinary angles. That covers most homes.
The top levels add a skim coat over the whole surface. That exists for one reason: critical lighting. Long corridors with wall washers, big windows throwing light sideways down a wall, or any gloss-level sheen will telegraph the slight difference in texture between a taped joint and the surrounding board. A skim coat removes that difference so the whole wall reads as one plane. This is why hotel corridors so often specify it.
The practical takeaway is to decide the finish level with the lighting and the paint sheen in front of you, not afterwards. It changes the labour and it changes the price, and it is far cheaper to agree up front than to skim a wall that has already been painted.