I recently got delayed on my flight form ORD to LGA on UA263 on 2/14/2020. I decided to look deeper into the cause of the delay. According to United, the plane was coming from OMA (UA1140). It was scheduled to be on time. This didn't make sense so I went back further, perhaps an earlier flight was delayed and the delay was just making its way through the system. The flight to OMA was in the air from ORD (UA1525) and was actually anticipated to arrive early! It didn't make sense that all the earlier flights were on time or early, but I was delayed. So I looked at the arrival/departure times. Flight UA1140 is scheduled to arrive at 5 PM but flight UA263 was scheduled to depart at 5 PM. It would be physically impossible for the flight to take off on time if it arrives on time. So, why do airlines do this? It is a change in plane mid-way through a daily schedule? Is it just bad scheduling? Is it something else? This info is coming from United's website, so it could be that the info itself isn't accurate.
You are looking at the aircraft scheduling at a single point of time, which won't show the full picture.
Most likely the aircraft that was planned to fly flight UA263 was delayed, or for some other reason unable to make it to ORD on time.
In order to handle this, the airline had to find another plane, and the first one they had available was the one operating UA1140. Clearly this resulted in a delay for UA263, but that delay would have been shorter than the delay if they had instead waited for the originally planned aircraft.
United's website will only show the "inbound" flight at the time you look, and by that stage they had already made the change, so you see an 'impossible' schedule. If you had, for example, looked yesterday you likely would have seen a different aircraft scheduled to fly that flight - with a suitable buffer between flights.
In practice, these "swaps" are generally far more complex than described above, and could involve multiple aircraft being moved between flights to minimize delays across all flights. This is especially true when there is bad weather or some other issue causing multiple flights to be delayed.
Update: I did a little digging, and this is indeed a multiple aircraft swap. The original issue was UA795 from Windsor Locks, CT which was delayed by over 9 hours. In order to manage this delay and minimize the impact on future flights, United swapped multiple aircraft across multiple flights. Your flight was originally planned to use the aircraft coming in as UA1573, but that flight was re-allocated to another flight, who's aircraft had in turn been re-allocated to another flight to cover the delay on UA795.
The end result of this is that a number of flights will be delayed a little (eg, you are due to arrive 25 minutes late), however that is better than a few flights being delayed >8 hours each as would otherwise occur.