I think we are confusing two things here, as there are two separate ways of doing this.

1. You can get an extra seat for health and safety reasons, for example for being oversized or when carrying a large instrument with you. You need to call to set this up, however in this case the extra seat is necessary and to my understanding will be guaranteed.
2. You can try and get a free seat next to you purely for convenience sake, which is not guaranteed, which you can purchase while checking-in. This can only be done if there are spare seats available and it is not guaranteed.

Pricing will also be different as in one case the airline will not be able to sell the seat, while in the other it is just selling a perishable good before it expires (In very crude terms, they are trying to get some money for an empty seat, which would otherwise remain empty anyway).