At the start of the game, you forces at the table are at the strongest they will be for the entire game.
The units that you selected, and the tactica planning to power them are the strongest.
Each game turn, through taking losses from your opponent's forces, bad tactical decision, and other in-game forces, you get weaker and weaker each turn.
Often to the point in the end game, where both sides barely have any models left.
Which means, at the start of the game, with 100% of our model resources we want to be bold and decisive- position our forces to be the strongest in terms of momentum and turn dominance so that even as we lose forces, we have superior position to help out.
The deployment phase is KEY.
For many, the start of the game begins on turn one- the *real* start of the game begin during deployment.
Based on the terrain in your deployment zone, mid field, and your opponent's side, how can you move out on turn one to gain an advantage for the terrain rules in your game- cover, elevation, additional armor saves, etc.
How can you move out to dominate the mid field?
One aspect of using deployment is the idea of deployment lockdown.
While you are moving your units out to good positions on the table, so is your opponent.
Deployment lockdown tactica is where you have a group of very fast moving units, which immediately move out turn one in to your opponent's deployment zone. By doing this, you encourage, almost force your opponent to destroy them in the shooting or attacking phase.
It encourages units in the deployment zone to stay put and attack- naturally with 100% of your opponent's units going against, say 5% of yours, the units you are sending out for the lockdown won't survive.
BUT that is OK, as long as the points, battle value, etc. currency used to buy the units isn't large- a small sacrifice for a massive first turn lead out of the deployment zone.