What is the point of having these standard sets of markers? I think I can rationalize standard sets of symbols for elements frequently encountered in maps across many fields (such as roads or socio-political boundaries), but I don't quite get the point of cheesy point markers (such as a body outline for a homicide scene). These symbols will never be able to replace a legend, so what is the point in having them standardized?
It's a great question. One standard set of iconic markers with which everyone in the world is familiar is their country's set of street signs: stop, yield, crossing ahead, etc. I hope the point of such standardization is immediately obvious.
Note that the actual meanings of many of the highway symbols are not intrinsic: they must be learned (especially the international symbols used in Europe, IMHO). Unlike words (which--although they can be ambiguous--tend to be well defined and understood by literate people), icons do not have any inherent meaning. Without standardization, their use for communication relies on the ability of the reader to derive meaning on-the-fly, as it were. This is done in two ways (and both operate, to some extent, at the same time in most maps):
Reference to a legend.
Constructing meanings based on personal and cultural experiences, guessing, etc.
The first can be time consuming: it slows down map reading, makes it error-prone, and inhibits the development of a spatial "gestalt" understanding that only a good map can provide. The second will always happen to some degree and is somewhat beyond the control of the map maker. To the extent that map readers have correctly learned a standard symbology, you can circumvent both problems, thereby assuring faster, richer, more reliable communication.
Alan M. MacEachren. How Maps Work: Representation, Visualization and Design, New York : Guilford Press, 1995.
... such as a body outline for a homicide scene...
Summarizing: Standard Symbology is used for fast map reading.
I think that's the point of standard symbology, when you look at it you know what it means. The legend is indispensable indeed. But when you look at a standard symbol like a "body outline" the very deep of your subconscious tell you what that means so you don't have to go to the legend and compare symbols to know their meanings.
Thanks to vector data digital mapping methods, it is easy to produce maps with various symbologies. However, the variety of maps symbologies remains quite low, because a map based on already well established symbologies is always preferred to an 'exotic' one (except for some very specific cases). The example of national topographic maps is a good example of standardized symbologies use. In the past, some national mapping agencies have investigated on the possibility to offer different symbologies for some of their popular map series. It appeared that the users prefer the plain old traditional symbols, because they have less difficulties to read maps based on legends they sometimes know since they are kids.
GIS techniques have also changed the deal: there are many new (compared to topographic data) thematic data used and available - it makes a lot of new stuff to put on the world map. Usually, there is no tradition for the symbology of such data, and some conventions are required. Such convention are useful only for users that need to read quickly and often the same kind of maps.
Also, calling something "standard" is often a kind of marketing strategy to promote and boost its reuse by others!
A lot of the standards work comes from the military (NATO etc) so that when you have half a dozen military forces bombing some where they can all agree what their friends and the enemy symbols are. It cuts down on so called friendly fire incidents.
A similar functionality is required when you have many local, state and federal law enforcement agencies trying to interact.
A concepts that helps from psychology: "extrinsic cognitive load". Translated: mental work that could be avoided in understanding (in this case a map) as compared to mental work that can't.
Having a custom icon for a homicide used only on your map leads to extrinsic cognitive load because you have to check the legend, match icon in legend then remember what it is. If you have a standard icon that you recognize from other maps you probably don't have to do any legend checking at all.
Another possible advantage of a standard set of icons is that they work well as a set. Designing a set of icons is very tricky (from personal experience!), you need to:
- Visually differentiate all icons from each other
- Allow groups of icons to be visually linked (e.g. in UK OS maps all churches are variations of black crosses, chapels = just cross, church with tower = square with black cross, church with steeple = circle with black cross)
- Avoid visual clutter where the map just looks a mess whilst not sacrificing  and .
- Avoid color blindness issues and other visual problems.
Of course not all standard sets are well designed but a good one saves so much time in terms of producing an effective map that you have to have a really really good reason not to use it.
Plus as soon as most people are using a standard set you get the same network effects as social networking - the more people use it the more difficult it becomes to justify doing something different.