Relations/Relations are not Categories
Dear Wikipedia contributor,
you may be used to each article in Wikipedia having at least one category. As soon as you create a new article in Wikipedia without a category, it will either immediately be marked for removal or added to a category. There are people who do nothing else all day than add meaningful categories to Wikipedia entries.
The "relations" we have in OpenStreetMap are not categories. They are meant to model a close (and usually local) relation between objects, for example: This entrance leads to that subway station, or: You cannot turn from this road into that road. We also use them to group fragments of a road, as in: These fifteen parts together make up so-and-so road. We do not, however, create relations that simply collect a loose group of somewhat related items. We don't do "Footways in East Anglia", we don't do "Scottish Lochs". As a Wikipedia contributor, you might feel the urge to find at least one relation for every object you touch - but please resist that urge. Our database is a spatial database; this means that it has intrinsic knowledge about the location of objects. If you want to know about all footways in East Anglia, simply pass in a bounding box of East Anglia and request all footways, and the collection is made for you on-the-fly. Anyone adding a footway just has to make sure it is in the right place and marked as a footway - the fact that this is in East Anglia does not have to be recorded because it is implicit.
So, again - please don't do things like "Footways in East Anglia".
But what about group relations that add information, you might ask, like "HSBC ATM machines"? Here, too, a relation is usually unnecessary; if the ATMs are tagged with something like "operator=HSBC" then anyone can easily extract all HSBC ATMs, you do not have to create a relation for that (this will only make editing more difficult and error-prone). Grouping relations really only make sense if the grouping is neither geographical (as discussed above) nor exclusive (like the HSBC example - the cash machine is unlikely to be operated by two different institutions at the same time).
A good example for a valid and useful grouping is the "route" relation, where multiple ways are connected to form a cycle route or a walking route or something else; a way may be part of any number of routes so this cannot be solved by tagging the way with "route=xxx".
Thank you for your understanding,
Those who invented relations.