You're discussing the technique of writing data to a device with the data being cached in memory before it's delivered to the device. In your two examples, the devices are a "hard disk" (which may use SSD technology for faster speed), and a network interface (usually wired or wireless Ethernet).
In the books/websites you have been reading, writes to a hard drive are cached in memory and a background thread moves the data from the cache to the physical disk. Writes to a network device are not cached to the same extent (there is cache, but it's much smaller). Why is this?
Hard drives are slow. This answer in a Redis mailing list thread about memory/disk speed illustrates the difference between the speeds of memory, SSD, and HD. Programs live in memory and when they work with code and variables and data structures in memory, they can run at memory speeds. But when programs must write to disk, the slower speed of the device - without buffering - brings the program to a screeching halt. As the comparison in that mailing list answer shows, it's the difference between about a second (writing to memory) and waiting several days (writing to HD) before you can do anything else. SSD is better, but still represents waits that will cripple any program. And it's not just writes to disk; reads suffer as badly.
It isn't tolerable to slow programs down so much, so for decades now operating systems have placed a memory buffer (cache) between disk and programs. Writes go into the memory buffer quickly, allowing the program to continue its function. Kernel thread(s) flush the buffered data to disk at the disk's slower rate, but it's done in the background where it won't hurt performance. When reading from disk, the operating system tries to read extra data into the read cache so the program that requests the data will wait less. It's not perfect, and when the program requests disk data that's not in the cache, the program has to wait for it.
With network devices there is also data buffering to help programs perform well, but the buffering is smaller and simpler. Books and websites don't go into the same amount of detail as they do with disk buffering.
