What is a Multi-tap Transformer?
First off, you might be wondering what a multi-tap transformer is. Well, it is a lighting transformer that has multiple connections – called taps – within the transformer which supplies power in the form of volts to your runs of wire. They are generally labelled 12V, 13V, 14V, and 15V in the landscape lighting world.
What is the Benefit?
In any large lighting system, or in any lighting system that requires long wire runs, voltage loss will occur. In some cases, for large lighting systems, using a thicker gauge wire can help reduce voltage loss. However, when it comes to long runs of wire, even with a thicker wire gauge, voltage loss will still happen. 
This is where having a multi-tap system comes in handy. The higher voltage taps (13V – 15V) supply long runs of wire with more power so lights near the end of the run receive enough voltage (12V) to operate effectively.
Ultimately, a multi-tap transformer provides your lighting design with flexibility for your output voltage requirements, by adjusting the available voltage to compensate for voltage drops as wire runs get longer.
What If I Don't Have One?
Whether you have a multi-tap or a traditional 12V transformer, it is important to remember to keep your system balanced, by having lights close to your transformer on one wire run, and those further away on another.