depends what you wanna edit,

You can use it for modding games, edtting config files and programs.

One example of hex editting is replacing the start button, with something like steve or what ever, thats a really odd think to change it to but I couldnt think of a better word/ name.

Hex is only representing the data in base to the 16 instead of base 10 which is decimal.

base 16 is used in programming becuase writing a memory address in base 16 tends to make things look less confused, and base 2 (binary) would be very difficult to interpret.

decimal is the way that we see things, cos this is then converted to ascii codes, which is what we see and is represented here

i2c