Why a top shot?

[Gobi King]

For trolling at high speed fish strike out of instinct (??) and leader is not really important,

Abrasion or bite resistance is what I use a leader for,

For trolling for salmon in great lakes,  I read that most fishermen use a mono leader of 50 ft or so, when they use a braid main line for the elasticity instead of using a snubber.

When I stream fish for salmon, I stopped using a leader, just tie my size 8 hook on to my main line (sufix clear mono), I could not tell the difference.

Edit:
Walleye/Salmon guys using planer boards have a mono topshot to attach their hardware to.
Dispsy and other use wire or braid behind a mono topshot to get the depth they need.

[alantani]

I keep seeing references about using a mono top shot. I understand what a top shot is, I don't get why you would use it.  What is the reason to put 100 to 150 yards of mono on top of braid?

it's a balancing act.  if you have a small reel and need line capacity, braid makes sense.  and likely you'll be using a fluorocarbon topshot, so a typical set up would be 300 to 400 yards of braid and a 25 yard topshot of fluoro.  for larger trolling reels, you might consider 400 to 800 yards of braid and perhaps 100 yards of mono.  that way you are keeping the braid year after year, and only changing out 100 or so yards of mono.  you need to figure out your "break even" point when comparing the the initial cost higher cost of the hundreds of yards of braid and then only the 100 yards of mono being changed every year, versus the cost of changing out an entire spool of mono every year, or maybe several times a year.

the other thing to consider is whether or not your reel has a power to weight ratio that makes sense for mono.  for an old guy like me, a 6/0 penn 114h has a lower power to weight ratio that is better suited for straight 60 to 80 pound mono and 15 to 25 pounds of drag.  factor in my age and the fact that i blew out my shoulder again.  a new makaira 50 two speed, on the other hand, can be rigged with 600 yards of 130 pound braid, a 25 yard topshot of 130 pound fluoro and have drag settings of 40 pounds at strike and 65 pounds at full. 

lots to consider........