Because, if the character cord is not designated in the page, when a Japanese referred to your page a letter is not displayed precisely that is not able to comprehend to us.
It is displayed like this. (This image was selected with respect to the fact that there is not intention. When it was searching a sample this is found firstly.)
To avoid this I recommend that the following line is written.
<META HTTP-EQUIV=Content-Type CONTENT="text/html; charset=iso-8859-1">
If this Tag is written in your page it is displayed as your intention also in the environment of Japanese.
Probably, this will fit to all the countries that use the Chinese characters.
Rewrite "iso-8859-1", in accordance with each environment.