In the context of a highly-branded WCM site, the default "chrome" (border and title) SharePoint wraps around your custom web parts can be ugly and inconvenient. At worst, it will make your site look SharePoint-y and require every web part added to a page to have its Chrome settings manually adjusted.
That's no way for a web part to behave in content management system but what to do?
In the past I've set the chrome type explicitly for each custom web part, either in the .webpart/.dwp file itself or programmatically:
<property name="ChromeType" type="chrometype">None</property>
this.ChromeType = PartChromeType.None;
To avoid doing this per web part (or cluttering up your nice little base web part class), you can set the PartChromeType property on the WebPartZone declared in your page layout:
<WebPartPages:WebPartZone id="wpzLeftColumn" runat="server" title="Left Column" PartChromeType="None">
Any web parts added to that zone with a default Chrome Type (aptly named "Default") will inherit this setting from the web part zone. Of course individual web parts can override this as required.