Sometimes, if you are very early for a flight and have no checked bags, and there is another flight on the same airline going to the same destination with empty seats available, you can politely ask the gate agent and they can add you to a standby list for those empty seats.
In general, I tend to think of this as being helpful for the airline. If they wind up having other passengers coming in late, missing connections, etc. they then have more flexibility to use that vacated seat on the later flight, to meet their obligations and/or increase customer satisfaction. Also, it sometimes happens that the original itinerary flight winds up delayed causing missed meetings, connections etc. while having the customer on the earlier flight means that they are more likely to be successful in that journey. Maybe the passenger can't rely on arriving at the airport early enough to take that earlier flight and so did not book it, but if conditions change and they did arrive (past security) in plenty of time (this may be because the first flight was delayed), and room is available and there are no checked bags, it seems that putting the passenger on the earlier flight (with the passenger's permission/request) would be mutually beneficial.
Some airlines allow this without any extra fees; some charge significant fees for it unless the passenger has elite frequent-flyer status. Those that charge significant fees indicate that the change causes significant extra costs for the airline. What are those costs? How do they compare to the potential benefits to the airline?
(For this question, assume that the ticket was booked in Economy class and the earlier flight has seats available in Economy class).