This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
struts.properties is mostly a legacy feature that allows you to specify configuration settings, or as they are called in struts.xml, constants. I believe, at this point, struts.properties is effectively deprecated, however, there are a few bugs open right now of settings that must go into this file and can't go into struts.xml.
Fun fact of the day: you can specify any setting you'd normally put in struts.properties or struts.xml in web.xml as a filter init parameter for the Struts 2 filter.