Three big reasons why you'll love a Sink Tip Fly Line.
While floating lines are simple to cast and simple to manage, not to mention by far the most popular fly fishing lines in use, sometimes they just don't do the job. That's because a floating line regulates how deep you can fish your fly.. but there are measure you can take to get down deeper.
Compared to full-sinking fly lines, other solutions are small potatoes.
If you like you can try solutions with a floating line, like using a longer or narrower leader then adding weight with a weighted fly, but such half-baked fixes are small potatoes when you consider what results you can get with sink-tip or full-sinking fly line.
Of course in doing so you will be raising the question, which of these fly lines is ideal for your individual fishing style? And you need to bear in mind that, wth a full-sinking fly line, the whole line will sinks as per the manufacturer's specs.
So when the whole body of the line sinks, this category of fly line is a superb option for achieving and maintaining a fly down throughout your retrieve... and take note, the best places are those with little or no current, like lakes, where you don't have to fix the line or keep track of the fly's drift.

What a sink-top line is, why you need to care & two BIG tips.
Put simply, a sink-tip line is constructed to ensure just a limited section of the line's end is allowed to sink. This sinking section comes in different tapers, configurations and lengths to cater for different fly fishing conditions. You two big things to think about are:
1. Mending & Line Control
A huge benefit sink-tip lines is the extra control they provide, especially in strong currents. As just a limited section of line sinks, the remainder that floats enables the fly fisher to easily mend and re-position the floating line and better control a drift or swing.
2. Casting Speed Control
In those moments when you need to get your fly down supe rfast while ensuring quick presentations, a sink-tip line with with a short sinking portion is the perfect option. Because with short sink-tip, you'll have less line under the water and therefore water drag.