Buy if you Plan to Get all the Call of Duty: Infinite Warfare DLC
Similar to the first reason to buy. If you know you will buy all the DLC you should jump in and buy the Call of Duty: Infinite Warfare Season Pass.
You can buy individual DLC packs that feature maps you really love, but it could limit your play with other gamers. With Call of Duty: Black Ops 3 we saw matchmaking limit gamers to those with the same sets of DLC.
If you only buy DLC 1 and DLC 3 because you like them, you won't be in matchmaking with anyone that has DLC 2. Over time this can limit the players you are connected with and it can be an even bigger issue on PC.