Pre-Order If You Want Instant Access to In-Game Content
If you want all of the game's early content you'll want to put in a pre-order. If you do, you'll some nice pre-order bonuses.
If you pre-order a copy of Star Wars Battlefront 2, standard or Deluxe Edition, you'll get Star Wars: The Last Jedi-themed looks for Kylo Ren and Rey. Star Wars: The Last Jedi is the next movie in the current trilogy of films.
On top of that, you'll get an ability modifier that upgrades the Millennium Falcon and epic ability modifiers for Rey and Kylo Ren.
Unfortunately, it looks like retailers have pulled the Yoda's Epic Lightsaber Mastery Star Card bonus from future pre-orders.
If you pre-order the Elite Trooper Deluxe Edition you'll get all of that plus upgraded versions of all Trooper classes, a A280-CFE blaster rifle instant weapon unlock, an enhanced scope modification, and an epic ability modifier.
If you want these bonuses right from the get go (you'll be able to earn some of them by playing), you'll want to pre-order the game.