Blogger's Template Designer, accessible via the "Design/Layout" tab when you log in to your Blogger's dashboard, provide a far wider range of basic designs and far more options for choosing the number of columns, column width, background images, footer structure, colour scheme, etc.
I used the "Template Designer" to modify one of the Blogger's Default Template to the current design of my blog, "OgbongeBlog.com".
If you download a Blogger template and it is compatible with Blogger's Template Designer, you will be able to modify the template easily. You will be able to change the colour scheme, the general structure of the template etc In short, it makes it so easy for you to make your blog design look so unique.
If you download a Blogger template that is not compatible with Blogger's Template Designer, you will not be able to modify the template with Blogger's Template Designer. You will have to know some basic html/css skills for you to be able to modify such template via "edit html".
To know if your template is compatible with Template Designer, Login to your Blogger dashboard, go to Design/Layout and choose "Template Designer".
Click the Background and Layout tabs (down the left hand side)
If you see a message like "Not applicale for this template", this means your current template is not compatible with Template Designer but if you see options to change the image, colour scheme and column structure, then your template is a Designer template i.e it is compatible with Template Designer.