If you use a comma in maths mode, you get a small space
after it; this space is inappropriate if the comma is being used
as a decimal separator. An easy solution to this problem is to
type (in maths mode) 3{,}14 instead of typing 3,14.
However, if your language's typographic rules require the comma as a
decimal separator, such usage can rapidly become extremely tiresome.
In such cases it is probably better to use the
icomma package. The packages ensures that there will be no
extra space after a comma, unless you type a space after it (as in
f(x, y), for instance), in which case the usual small space after
the comma appears.