yep, what carol says.
sometimes you want to print to a postscript printer if it supports it.
for example if you are printing .ps postscript papers from the web.
so occasionally you will install 2 printer drivers, creating two windows visible printers (one pcl and another postscript), both driving the same physical printer.
i do this with my hp2200. printer experts might be able to shed more light on this, i just know from experimentation that sometimes tis better to use the postscript driver and sometimes better to use pcl (never seen much dif between pcl 5 and 6), so i install both as different ways of printing to the same printer.
that can also be useful if you want to configure like multiple print drivers for different settings (quality, doublesided, etc). just because you have 1 physical printer doesnt mean you cant add it 5 times with different settings and treat it like 5 printers as far as windows is concerned.