Why? Because it suits a variety of different frame styles, which is what a lot of my Retrograde clients have. And also because it takes away the pressure of having your art and photos spaced perfectly or aligned. For me, it's more important to get those pieces of art out of the storage boxes and up on the walls. However, the process isn't totally random. I like to use key lines to arrange art on walls. (That just means lining up certain edges of your pieces.)
Here are some examples (the key lines are marked in red):
In this example (above), you can see that the key line can also go down the center of several pieces.
Top and bottom key lines are easy ways to line things up, especially if you're really keen on maintaining order.
For this family gallery mockup, I layed out a 4' x 8' area of white space to simulate the portion of wall I was working with in a single family home. (When planning a grouping of so many photos or art, it can help to lay things out on the floor first...)
It's like a big puzzle, so go ahead and give it a try--play around! Like I said before, it's better than keeping your art and photos in storage. And if you mess up, just invest in a tub of spackle from your local hardware store to fill in the holes ;-)