We want to keep dissolved oxygen out of condensate. It changes the pH of the water. This will contribute to corrosion problems in the system. Especially the boiler. Boilers like a ph of somewhere between 10 and 11.
Hot water is almost the perfect solvent.
Give it enough time and it will dissolve anything. Remember that boilers have to last thirty years or more. This means that water has plenty of time to do its damage. It is the oxygen in the condensate that makes condensate a strong oxidizing agent that can attack metals. Some carbon seal faces can be attacked by high oxygen levels in the condensate.