Unicode often counts the same symbol (glyph) as two or more different characters. For example, Ω is U+03A9 when it represents the Greek letter omega and U+2126 when it represents Ohms, the unit of electrical resistance. Similarly, M is U+004D when it’s used as a Latin letter but U+216F when it’s used as the Roman numeral for 1,000.
The purpose of such distinctions is to capture semantic differences. One example of how this could be useful is increased accessibility. A text-to-speech reader should pronounce things the same way people do. When such software sees “a 25 Ω resistor” it should say “a twenty five Ohm resistor” and not “a twenty five uppercase omega resistor,” just as a person would. 
Making text more accessible to the blind helps everyone else as well. For example, it makes the text more accessible to search engines as well. As Elliotte Rusty Harold points out in Refactoring HTML:
Wheelchair ramps are far more commonly used by parents with strollers, students with bicycles, and delivery people with hand trucks than they are by people in wheelchairs. When properly done, increasing accessibility for the disabled increases accessibility for everyone.
However, there are practical limits to how many semantic distinctions Unicode can make without becoming impossibly large, and so the standard is full of compromises. It can be quite difficult to decide when two uses of the same glyph should correspond to separate characters, and no standard could satisfy everyone.
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 Someone may discover that when I wrote “a 25 Ω resistor” above, I actually used an Omega (Ω, U+03A9) rather than an Ohm character (Ω, U+2126). That’s because font support for Unicode is disappointing. If I had used the technically correct Ohm character, some people would not be able to see it. Ironically, this would make the text less accessible.
On my Android phone, I can see Ω (Ohm) but I cannot see Ⅿ (Roman numeral M) because the installed fonts have a glyph for the former but not the latter.
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This post first appeared on Symbolism, a blog that I’ve now shut down.