Retainers used to be sort of a manageable NPC. You gave them stuff to be in their inventory which they could then go sell in a giant marketplace you actually had to visit with your character to buy from. There were HUNDREDS of peoples retainers standing in a giant hall and you had to check each one to buy stuff from people.
Luckily this was revamped to what we have now where there's a GUI (the marketboard) and they also added some fun functions to the retainers like giving them jobs and "ventures" or quests to go on which might reward you with random items. However the old system is STILL THERE, buried under what they've layed on top. This is why our storage is linked to them. It's not a great reason and surely could use some redoing, but that's why.
What you can assume when looking up housing items is that if it doesn't SAY it does something in the description, it's decorative. Item descriptions are very specific about what they do and just for future information no item you can buy for your house/apartment/FCroom adds storage space.