What is the difference between 蟹 and 螃蟹?
Many individual characters are said to mean something even though they are not (or rarely) used as independent words. I think user6065 pretty much summarizes what is going on.


  although 蟹 can be used by itself, it is also part of many compounds, e.g. 螃蟹 crab， 龙蟹 lobster (for more see 小马词典），螃蟹 would seem to be a more precise unmistakable term for crab


螃蟹 is preferred because it is easily identifiable as meaning "crab", whereas xiè uttered in a different context could mean many things. There is even a famous play on words on 河蟹 illustrating this.

There is a preference in Mandarin for two syllable words where potential amiguity may arise. Standard Chinese (普通话) is a relatively innovative Chinese dialect in that it has lost many distinctions found in other Chinese dialects (for instance, in Cantonese, there are many more possible consonant sounds at the end of syllables; Mandarin only allows n and ng. Additionally, Standard Chinese has less tones than many dialects (once again, c.f. Cantonese). But speakers have more strategies beyond more complicated syllables and more tones to distinguish words. Words may be made longer, for example:


孩子 "child"
茄子 "eggplant"
狮子 "lion"


In the above words, the final character 子 doesn't contribute any meaning but rather serves to differentiate these words from others with the same first syllable.

Additionally, there is a smallish class of Chinese words that were always (as far as we can tell) polysyllabic. The most well-know of these is likely 蝴蝶 "butterfly", where neither the first nor second character are meaninful on their own, but they are now both understood to mean "butterfly" by virtue of mutual association. Perhaps 螃蟹 falls under this class as well.