I bought this for kits that are able to fit these in for the lighting gimmick.. You might have seen it on PG Strike Freedom and MG Jesta have lighted up eyes. Some of the next upcoming kits like Nu Gundam, Exia and 00 Raiser are able to fit those LED units in.
I'm getting the batteries for those LED units so once I have those I can do the lighting gimmick on Nu Gundam which I'm currently building XD
Cost is 880 yen, which is around $11 in SGD. But I wouldn't suggest you to buy this online alone. If you're buying through HLJ you might consider package it together with your other kits so that the shipping fee wouldn't be much of a burden for you.