Very handy - I connect the ones I need for a particular event and can keep track of them more easily.
Plus if a little more security is needed I can easily attach this to a neck cord to wear under a shirt.
I really like my ID Card Case where I store the bare minimum (driver license, credit card, and maybe some cash). Great for a quick run to the store (w/o my purse) or when I go for a run. These are great starter wallets for kids. So I brought 2 of them for my soon to be teenagers when they need to carry a wallet.
Other ID holders are too tight. This is ideal because it fits ID, transportation cards with some room to spare. Bonus a dividing inner wall. I put my work keys inside, all-in-one. And, I can attach to a lanyard. Will buy more.