The Coldkey wallet is like a piggy bank for your crypto-currency funds.
When you send some coins to a Coldkey, your funds are not on an electronic device anymore. They can be accessed only if you physically have the card.
You can easily load the card how many times and often as you want, but in order to spend the funds you need to physically alter the card.
Coldkey takes off much burden from you.
You don’t need to keep track of the best IT security practices when storing your crypto-funds on Coldkeys.
If your computer is hacked and you use Coldkeys to hold your coins, then they are safe and will remain on the wallet card.
Hackers steal crypto-coin funds mostly by penetrating systems and compromising the private key.
(The secret data necessary to spend crypto-funds)
This has and can happen to exchanges too ! It already happened to MtGox, Bitfinex, QuadrigaCX, Binance …
The Coldkey’s private key exists only on the card itself, and there is no copy of it so you control the funds.