"Recognize, evaluate, and respond to the dynamic circumstances within and surrounding the project in a holistic way to positively affect project performance." PMBOK, pg. 37
I don't know how many times the word "system" is used in section 3.5, but it's a LOT! Projects, though done to deliver specific scope and benefits, usually interact with other areas and systems/projects within an organization. You can't view a project in isolation. This tunnel vision can lead to missing other important elements. That's why it's also important to know how this fits into organizational strategy so you understand the bigger picture (3.4 talks about this).
For example, I was recently leading a reporting effort. We were ready to start extracting data. Then, someone mentioned one of our source systems was going to change because of a separate infrastructure project. After asking questions and taking a larger view of the landscape, we uncovered that our data mapping would be invalid within a couple months. Because of this interaction, we were able to pivot and avoid costly rework and possible reporting issues down the road.
I used to spend a lot of time in airports pre-COVID, and something you heard and read frequently was TSA's "If you see something, say something" message. As project leaders, we need to heed that advice and ask our teams to do so as well. Because our project teams come from other functional areas, they can be privy to information you're not. Advocate that if they see or hear something that might impact the project, they should say something to you. Potential interactions with other systems can be identified early and mitigated. I've had more than a few instances where a team member heard something, brought it to my attention, and then asked a lot of questions to gauge impacts.
PMBOK mentions having "empathy with business areas" as a skill that supports a systems view. I would expand that to having solid relationships and trust built at multiple levels. If someone, either a project team member or other stakeholder, hears of a possible issue outside of the project that could have an impact, they need to feel comfortable coming to you and bringing it up. If you get upset at the slightest potential change, people will be uncomfortable talking about it.
Having a systems view requires not only thinking tactically about your project, but also having a big picture view of all the interactions taking place around you. Knowing a change in something can (and usually will) have an impact somewhere else can uncover potential risks before they become costly issues. Build relationships that allow for potential impacts to be brought up and mitigated quickly. And remember, if you see something, say (or at least ask) something!