Sometimes, when boarding is just finished, flight attendants call for a passenger that must go see the flight attendants. What can be the reason for such an action? Is this just an extra check when the computers cannot tell if someone is already in the plane? Is there another reason?
When you're boarding, the person taking your boarding pass from you and then giving some or all of it back scans it or swipes it to indicate that you are actually getting on the plane. In response the computer beeps and provides some more information. Usually, that information is just "ok, seat 12A has boarded." But sometimes it is "ask the passenger for their upgrade coupon" or "the passenger hasn't demonstrated they can fly to this destination" or "the passenger has been assigned a different seat." The gate agent is supposed to react to this information and ask you to step aside and talk to the colleague, or hand you a new boarding pass (this has happened to me) or the like. But sometimes they get absent minded and they don't. So someone goes onto the plane and completes the business that the gate agent missed.
Also once, after I was sitting, a gate agent came onto the plane, knelt next to me and said quietly "I got you a better seat, come with me" and moved me up to business. There was a note on my file that I was flying for a funeral.
A common reason for this at least in the US is when a passenger buys duty free, but forgets to pick up their purchases.
In the US, duty free merchandise is usually not given to the purchaser right away, as there is not always a separation between domestic and international departure gates, and passengers can even exit the secure area. The purchases are set aside, and brought to the gate, where they are given to passengers in the jetway.
It often happens that someone will have made a purchase and still go right past the duty free collection point. The passenger will then be called back to the gate to pick up their purchases.
There are probably dozens of other reasons, which may include issues with checked-in luggage, last minute upgrades, or even the occasional bump off the flight (especially for “free” passengers such as staff family who ride on a low priority basis).