Pay for your inventory.

[Evergrey]

And that space will be immediatly filled with Heavensward stuff like "Oh, nice lv55 bow I just dropped there. I'll hold on to it for when I'll be leveling my Bard".
Plus new mats.
Plus new Glamour items.
Plus new potential events items.
Plus all these desynth stuff you'll need to level up desynth.
Plus new housing items that don't fit in your house just yet.
Plus every leveling gear you'll need for non-50 classes/jobs.

And that is why you clear out the old stuff to get room for the new.