Windows decals are highly effective for many reasons, but mostly because they will be seen by everyone who goes past your building - and even if they don't register your company logo on a conscious level when they walk past, they will still likely register it unconsciously and think abut your business next time they're looking for your service. At the same time if you have a high street store or restaurant then they can be used to bring people into your company. You can this way advertise things such as special offers and deals and generally make your business look more appealing than the one next door. In this way it improves not only the advertising and awareness of your building, but also the appearance of the building itself.

On the other hand if you run a company with a block of offices for example then you will not have people walking past your windows on a regular basis and will instead be broadcasting your message across large distances.