I am sure they will help, and I have had positive experience with United the 1 time I had to make a change in the past year.

However, it is not always a straightforward solution. You are a family of 4, and the airline allots. you 2 + 2 (middle+aisle on one side, and aisle+middle on the other side) as that is efficient, but you would rather have window+middle+aisle + aisle. Or maybe vice versa.

basically an algorithm may not meet your needs, and while the airline can help adjust, it is an overhead in terms of personnel costs, and also if you decide to pick seats last minute when most of the seats are already allocated.