Pros: easy to use, easy to see
Cons: some folks get a little carried away
No matter what number of colors Irwin flagging tape may come in - and I have four or five of them myself - the count of uses for this versatile material far exceeds it. We've probably all seen strands of the tape alongside the road, tied to fences, or looped through the branches of a tree. They've probably been placed there to identify survey points ("find iron rod at...") or to mark a trail. I first used flag tape to mark my path up the cliff face as I did geology field work (it's less obtrusive and permanent than the brilliant yellow spray paint some classmates used), but I've since gotten in the habit of keeping several rolls around.
The tape can be as unobtrusive as a dark green or as noticeable as neon orange or shocking pink; make your choice. Whatever the color, though, the 2-mil PVC (poly-vinyl chloride) plastic is ideal for its task: it tears easily (even when you're wearing gloves), knots securely for tying to stakes or branches, and readily takes notes printed with Sharpie® markers. At more than an inch wide, the brightly-colored flags are visible from hundreds of feet away, and maintain their color for months or even years.
Besides my geology field work, I've used my flagging tape for laying out fence lines and marking locations. It ties onto a string to mark the locations of posts and other features, and also to make strings and wires visible when hung across open spaces. It's also proven useful for marking trails, whether one's out cross-country skiing or trying to navigate a maze of unmarked roads out in the mountains or desert.
Carry a roll (or two) with you whenever you're out in the back country: who knows when you'll need to find your way home in an area where your cell phone can't get any bars!