Usually, when you see coir in the soil around pot plants, it's not just on the top, it's actually a component of the soil/potting compost. It's used because it's an organic waste product, cuts down on the use of peat, has excellent water retaining properties and contains a mixture of coarse and fine fibres to hold a little air in its pore spaces, making a lighter, more aerated compost. It is, though, poor at holding nutrients, so is usually mixed with other materials to make a growing medium. It's used more often for some plants than others - palms often have a coir mix compost.
I suppose it's possible that the plants you saw, which appeared to have a coir 'topping', may have had one just for decorative purposes and possibly to slightly reduce the need for watering. Often, though, in the UK, a topping or mulch on top of a pot is done in commercial enterprise to cover up the fact that the soil beneath isn't quite right - maybe the plant's been around a while and has grown moss and lichen on top of the compost, and a mulch covers all that up. It's quicker and cheaper to apply a handful of something to the top of the pot than it is to pay someone to clean up the pots properly. This does not apply to alpines, though, which are almost always sold with a top dressing of chippings or grit.