The advantages of doing this are obvious: you can create a leader to specific tasks of fishing. You can dictate how quickly or slowly a leader will turn over during the cast. You can make a leader to perform for specific conditions & locations, too. The possibilities are endless. Of course you have to know, or find through trial & error, the correct formula to make the leaders you want & there's the trick.
Through the years many folks have developed different formula for leaders for different uses & the angler's preference comes into play, too. That's why you might find different leader formula designed for the same goal. It's all personal preference.
For example, shown below is one leader formula for dry fly fishing by the legendary "dean of fly fishing" George Harvey. Here Mr. Harvey has designed a leader to land on the water with some slack in it in order to prevent drag from setting in on his line too soon. All sections of leader are usually connected together with a nail knot.
10" of stiff .017" thick nylon, to this add:
18" to 20" sections each of
Then attach tippets of:
12" of 2x
18" of 3x
22" of 4x
This makes a 9 ft 4 x leader.
It might be a good idea to get a set of calipers or a micrometer with either a regular or digital read out so you can measure existing leaders in your vest. Then you'll have something to compare your leaders with as you come up with new designs.
Who knows, you might just invent your new favorite leader for your favorite section of stream that will help you to catch that pesky brown trout that's been eluding you? At the very least it's a fun way to spend a cold winter's evening.