You can always find items you rarely or have never used, and can trim them out of your pack, or, realize you used something up on your last trip out, and it needs replacing.

Image source: http://www.tumblr.com/tagged/hiking%20gear

More importantly, you might also find after an inspection of your gear, that a piece of your gear is broken or is malfunctioning, such as your bladder is punctured from the tossing your pack into your car after your last trip, and your trekking poles which you jammed in there pierced it. You do not want to find out one morning, as you are scrambling to get out of the house and to the trailhead one morning, when filling up your bladder, that it is unusable.

Because I am a bit of a gear head, I might even go through my pack a couple times within a week, and at other times, it is much longer in between reviews of my gear.