According to an official blog post by Ubisoft, the modern-day portions of Assassin’s Creed IV are kind of a side attraction. The idea behind these segments is that you are playing as yourself, hired by Abstergo Entertainment to sift through history in the name of “game testing.” Assassin’s Creed fans likely remember that Abstergo Entertainment is essentially a front for the Templars. So it’s up to you to figure out why Abstergo wants you to look into the history of Edward Kenway, the main character of Assassin’s Creed IV .
That is, if you want to. There is a ton of content in the modern-day sections of the game, including computers to hack, meetings to spy on, an employee manual to read, and more. In fact, all of Abstergo is basically a sort of satire on the video game industry.
However, gamers can pretty much decide to ignore all of this if they want to. If you were to play all of the optional content in present day, you’d get a solid three to five hours of gameplay out of it. However, gamers can simply choose to keep their mouth shut and stick to the past if they like, in which case they will have to spend about 20 minutes in the real world–tops.
Source: Ubiblog