Why does the i386 version of most software get installed along with the x86_64 version on a CentOS 5.8 64-bit system?

# yum install php-xml Loaded plugins: fastestmirror Loading mirror speeds from cached hostfile Setting up Install Process Package php-xml-5.3.12-5.el5.art.x86_64 already installed and latest version Package php-xml-5.3.12-5.el5.art.i386 already installed and latest version Nothing to do 

Why does the i386 version of most software get installed, along with the x86_64 version, on a CentOS 64-bit system?

# yum install php-xml Loaded plugins: fastestmirror Loading mirror speeds from cached hostfile Setting up Install Process Package php-xml-5.3.12-5.el5.art.x86_64 already installed and latest version Package php-xml-5.3.12-5.el5.art.i386 already installed and latest version Nothing to do 

Why does the i386 version of most software gets installed along with the x86_64 version on a 64 bit system (CentOS 5.8) ?

yum install php-xml Loaded plugins: fastestmirror Loading mirror speeds from cached hostfile . . .  Setting up Install Process Package php-xml-5.3.12-5.el5.art.x86_64 already installed and latest version Package php-xml-5.3.12-5.el5.art.i386 already installed and latest version Nothing to do 

Why does the i386 version of most software get installed along with the x86_64 version on a CentOS 5.8 64-bit system?

# yum install php-xml Loaded plugins: fastestmirror Loading mirror speeds from cached hostfile Setting up Install Process Package php-xml-5.3.12-5.el5.art.x86_64 already installed and latest version Package php-xml-5.3.12-5.el5.art.i386 already installed and latest version Nothing to do