OS OpenSpace provides a nice JavaScript API for adding maps to you own website, much like Google Maps and the one from Microsoft that changes its name every week (called Bing Maps at time of writing) but it misses a few things like aerial photography and maps outside the UK. So is there any reason to use it? The main reason I can see is to use the boundary layers they provide that aren't provided by the other map providers. This page shows how to do that. Select a layer from the dropdown and it should be displayed in the map below.
Boundary :