You can certainly use only a liqueur/simple syrup glaze over plain pound cake or bundt cakes, etc... But for decorated cakes, simple syrup soaks are used to help maintain moisture (usually in scratch) cakes.

Everthing you ever wanted to know about making and using simple syrups:

How to Apply Syrup:
(Includes recipe.)

http://www.dianasdesserts.com/index.cfm/fuseaction/recipes.recipeListing/filter/dianas/recipeID/685/Recipe.cfm

Cool Syrup Applied to Cooled Cake:

http://www.akiskitchen.ca/recipes_cakes/four_lwc_orange_custard_chodippped_ladyfingers/fourl_lwc_orcust_ladyfingers_1.html

Cool Syrup Applied to Hot Cake:

http://www.akiskitchen.ca/recipes_cakes/ravani/ravani_1.html

How Simple Syrups Add Moisture:

http://en.allexperts.com/q/Desserts-747/2008/11/simple-syrup.htm

http://blog.josephhall.com/2006/12/simple-syrup.html

How CC members use simple syrup, opinions:

http://forum.cakecentral.com/cake-decorating-ftopict-55178-.html

HTH