User blog comment:FairyRaven/Fairy Tail Chapter 429 & 430: Code Blue & Purification Ritual, Review and Commentary/@comment-3424593-20150420131607/@comment-7418318-20150424064254

I can believe the 100 armours. The first OVA has portrayed Erza as an armour collector, just like other people collect coins of post marks.

So my guess would be that some of her armours are for practical use (Heart Kreuz, Elemental Empress armours, Amarura Fairy, ect) but others are just part of her collection.

Then there's also the Nakagami Armour, which suggests that magical items of a certain strength need a strong enough mage to be wielded properly. If this is the rule rather than the sole exception it could be that Erza used to be unable to wear her current sets of armours earlier in her life and thus had to opt for less powerful ones. If she kept those old (now more or less obsolete) armours instead of throwing or giving them away, I can see quite a number piling up, even if she currently uses only 10 or so.