Take a look at the following list:
One might say, for instance, that:
\mathitare for math mode, while the others are for normal mode.
\itare for plain TeX and obsolete, while the others are for LaTeX.
However, my real question is: What was the need to redefine a command? Couldn't the normal mode commands be carried over to the math mode, or plain TeX commands be carried over to LaTeX?
PS: Perhaps there are other differences which I don't know. Please mention them in your answer, too.