So let's start with what "Reach" is. It is the ability to engage another model in melee from 2" away or in some cases further. The base melee range of models is only 0.5", so this can be a major benefit in adding threat range to charges, and engaging past a screening model to engage another behind it.
For accurate examples, I've used the Vassal module to illustrate some tactics and uses of this ability.
The Line Formation:
|Notice the broken command rng when trying to screen.|
The Box formation is a basic formation that is versatile and can quickly shift into other shapes as needed. As such, this is typically how I begin the game, as opposed to the line formation. This formation sacrifices offensive punch for versatility.
|Open Box with kill zone in the middle|
In historical relevance, a triangular formation was often considered a Spearhead. I have chosen not to call this formation by that name because it's really upside down in this game.
This is usually the formation I'll use when I want to run and engage, since once engaged, the rear models are primed to screen the flanks and charge later.
|Only one model in the unit remains free to respond however they want|
Running and Engaging:
At times, you'll want to run and engage. In this case, you're either trying to tie up range models, who likely have only a 0.5" melee range, or it is your last resort. Luckily, reach models are brilliantly made for this.
Whenever possible attempt this:
Essentially, using the screening techniques above, you should be able to do a heck of a lot of engaging with only 3 models, while the others are able to screen the flanks to help protect the unit. This should minimize casualties, or at least make it a real mess for your opponent.