Cardinal directions on signage is mostly an American thing, hardly any other country except perhaps Canada consistently uses cardinal directions on their signage.
However I've seen some photos of Chinese reassurance markers having cardinal directions.
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Excuse my ignorance then. Thought the lacking cardinal directions is more of an exception instead of the norm. I know a general idea of where Chinese cities are in relation to the location of the sign, so I could do with it. Just wondering about people that are unfamiliar to an area (especially with a foreign language) and trying to drive on the freeways.
Though yes, the reassurance markers have cardinal directions on it.
In China's system, the endpoints are set already in name too (in addition to the mileposts), and there's no changing it.
One benefit of such a system is that there is no argument as to whether route number N should or should not serve city X. If a route is designated as going from A to B, then presumably its route should always be based on what's the best route from A to B, and not whether it serves or doesn't serve some intermediate point C, D, E, etc. For example, US roadgeeks often debate whether route X should go through a city center or use a bypass. I think the answer should really be based on what we want traffic on that route to use, so designating the purpose of a route helps with that.
This is the case for most roads, though there are exceptions. G15 is an example. As mentioned above, the two endpoints are Shenyang and Haikou, except both water crossings near each endpoint is incomplete, so I'll look at between Guangzhou and Lianyungang. Fastest routing between those two points is a combination of the inland G45-G60-G70-G50-G42-G25 routing, and using G15 all the way isn't an option as it's the coastal route of China's system. So even with both incomplete crossings done, G15 is not the fastest/shortest route between Shenyang and Haikou due to its coastal routing and serving the coastal population centers like Shenzhen, Xiamen, Fuzhou, Wenzhou, Shanghai on the way.
This one is less noticeable, but the G25-G15 combination between Beijing and Shanghai is faster than G2, the Beijing-Shanghai Expressway, by about half an hour.
As for serving the city center, the two digit G expressways generally bypass the city center to some extent, with spurs (national, provincial, unnumbered just named), arterials or ring roads serving it from the expressway.