I work in IT, and if you go the extra mile for a person one time they often expect that to be the norm from that point forward and may even tell other people about it who will then expect it to be the norm for them as well, and it just becomes extra work that's expected of you on top of your normal job duties, and if you complain or try to stop doing it you'll be seen as not a team player or lazy for trying to do less.
Or if you agree to do a favor for a co-worker one time, and then they ask a second and third time, and they start asking more frequently until eventually they're just coming to you every single time and it ends with you basically doing part of their job for them.
As you mentioned it can be helpful to do this because you do get those connections and influences with other people, you just have to decide if it's worth the effort to you personally to be doing what amounts the extra unpaid work to get those connections and influence.