The cryptocurrencies are new technology revolutions, and develop for some years. Many changes happened in the past. In the very first stage of cryptos, every one believed decentralized, anonymous, but these excellent qualities changed siliently. Especially, much more people deposit their coins into the third party platforms, which are decentralized services, at first we believed them as they were trust, but they were not really trust over time. I have experienced that I can't withdraw my coins deposited on the website after I deposit too much coins, their website "freeze" my coins and say I need to submit many evidences. But I still can't withdraw them after I submitted many and many evidences, at the same time I lose my anoymous. So store your coins into other's hands are very dangerous.
Many popular coins have super large blockchain size, because of which you want to store your coins in other one's hands to decrease your storage pain. That's it one reason, the third party centralized services have enough confidences to invade your privacy, freeze your coins or even take your coins. If you are using such a coin, you can use electrum wallet and make deep investigate into the electrum services, or other choices of not use such coins and turn into other small blockchain size coins, instead of store your coins in a dangerous centralized place.
If you want to buy some coins from the market, such as you want to buy some spartancoin, you can deposit your coins to the market, exchange, and withdraw them in time, never put too much coins into the market and withdraw the exchanged coins as quickly as possible, with these two methods you can decrease your risk.
Cryptocurrencies are decentralized, but it not means they can't not be used in centralized scenario, so some evil people would make a cheat trap to take your coins. Remember that you must keep in mind you do not trust others too simply.
With the popularity of cryptocurrencies, as things progress, more and more people know digital coins, many dangerous people want to kill anonymity, decentralized, freedom, they want to take other's wealth. I am not threatening you, really, the world is not safe, never safe, you should take your coins under your control, that only decentralized feature can let you keep your coins not be rubbed.
SpartanCoin is such a decentralized coin, its block chain size is very small, only about 340M currently. You can use the full block chain spartancoin wallet with confidence, no one can take your SPN in your hard wallet.