One of the greatest things about the Tau in Warhammer 40,000 is the long range weaponry and the mobility of the units carrying them. This combination allows you, the Tau player, an ability which most other armies lack, the ability to focus your firepower where it is needed. Good Tau players will have assigned roles for each unit and will use the mobility of those units to bring that firepower where is needed. For example, if you have a unit of Crisis Suits with twin-linked missile pods(Deathrains), this unit is best at killing rhinos and other light vehicles. The jump-shoot-jump ability of the their suits gives you the ability to choose your targets where other static units would simply be force to shoot at whatever is in range. Crisis Suits on the other hand can move to their targets without forfeiting firepower.
I tend to steer clear of static units in my army list for this very reason. Too easily your opponent can negate their effect by staying out of range. The sole exception to this are my Broadsides, but their weapons have a 72 inch range so set up in the middle, nothing is out of range.
For my army, I generally tend to assign my Deathrains to taking out transports, then my Fire Knife Suits target marines or monstrous creatures. Hammerheads take out troops and Broadside fire at tanks, Piranhas move out to target AV14. Each unit has an assigned role and my mobility and range, allows me to move to those targets, regardless of terrain and my enemies deployment. No unit should ever be left without purpose and the means to achieve that purpose, i.e. mobility or range. This is economizing your force.
Economy of force is the principle of employing all available combat power in the most effective way possible. In Warhammer 40,000 this means that you are firing weapons at what ever they are best at firing at. The easiest example is firing a meltagun at vehicles instead of a unit of 30 Orks or firing your burstcannons, wrongly, at a Rhino and hoping for a 6 on the armor roll. Simple concept really that comes naturally to most players. It's important to achieve this economy of force whatever army you are playing, but more so with Tau as we cannot roll people in the assault or field any uber-units that can take all comers.
Along with achieving an economy of force, the Tau's mobility also give you the ability to bring your firepower to bear on the decisive point, be a cluster of objectives on the board, your opponents main attack on your base in capture and control, or those easy kill points in your opponent's army.