A Campaign is Important
Lets get the obvious one out first. The first Titanfall fell away due to a lack of content, and not having a single-player mode meant that players didn’t get any background on the world of Titanfall or the relationship between a pilot and their titan. A campaign also acts as a glorified tutorial, without meaning to sound disparaging. That is the case even more so in Titanfall since there are two core mechanics that are both first person shooter based, yet feel completely different. A campaign allows you to gauge the strengths and weaknesses of each play type whilst determining what you, as a player, are good at.
Titanfall’s story also adds vital context to the world. The campaign makes clear how the role of titans and pilots are connected, and that follows over to multiplayer. This isn’t only the case for Titanfall 2 — a campaign is important in any game that features a new world with the potential to tell an interesting story. Respawn clearly listened to all the people that asked for a single-player story, and they followed through with a strong campaign that adds a lot to Titanfall as an overall package.