A tilt & slide window does exactly what the name suggests — tilt the top of the sash inward slightly for background ventilation, or slide the entire panel open for full access. Most window types only give you one mode of opening; this gives you two from the same unit.
On a balcony or terrace, that flexibility matters more than it might in an interior room. The tilt position lets air circulate without fully opening the space — useful overnight or when the room is unoccupied — while the full slide gives unobstructed access when it's time to actually use the outdoor space.
The mechanism also tends to seal better in the tilt position than a fully open slider would in an equivalent partial-open state, since the tilt is a deliberate, engineered position rather than a sash left partway along a track.
For any room that opens onto usable outdoor space — as opposed to a window that's mostly for light and view — the added mechanism is generally worth it for the flexibility it buys.

