From the tippet to the butt section, your leader is vital to catching fish.
After all, it's what connects you TO the fish! No matter what the strength or stiffness is, or how good the advertised "abrasion resistance" sounds, it can still be prone to damage. While these wounds can range from a simple abrasion to a small slice, they all have one thing in common—they weaken the line. I think everyone reading this can agree that the very last thing you want is a weakened leader, especially if hooked up to the fish of a lifetime! For this reason I make it a habit to check my leader both often and carefully.
I fish for snook a lot here in Florida. For anyone that hasn't caught one before, let me tell you that they can be extremely abrasive to a leader. In fact, when fishing for them a shock tippet is basically a necessity to protect against excess wear. Many times, even a little snook can really chafe the tippet just above the fly. It often looks like someone has literally run sandpaper across those few inches of line, and when that happens I always snip off the damaged portion (or replace the shock tippet if it's getting too short) and tie my fly back on. Not only is this line weakened, but when the line is chafed it can lose some of its translucency, which in my view makes it more visible to the fish. Yes, it can be an annoyance to constantly re-tie (sometimes after EVERY fish), but it's also good insurance.
To check the leader, I will usually just pinch the line between my thumb and forefinger and run these fingers along the line (while also visually looking at the line) to feel for any abnormalities. If I see or feel something that catches my attention, I'll give it a closer look. Sometimes it's a stubborn piece of moss fused to the line or a leader knot, other times it may be actual damage. Occasionally, there may even be a tiny wind-knot (basically an overhand knot) caused by a sloppy cast. These can weaken your line, too!
Even if not fishing for particularly "abrasive" or "toothy" fish, it pays to check the leader.
It can sustain damage from any number of things like hitting brush during a cast, rubbing against submerged rocks, or being stepped on or dragged across the ground. Although any portion of a leader can get damaged, the tippet is the most vulnerable since it's the lightest section and the closest to the fly. After catching a fish of ANY kind, I always give the tippet a quick inspection and at least glance at the rest of the leader. When fighting a fish, this is prime time for damage to occur or junk to collect along the leader's length. A quick couple of seconds can prevent a disastrous break-off later on!
Being vigilant on the water doesn't mean just being in tune with your environment and the fish. Take care of your gear and it'll take care of you!