The benefit of a USD wallet on Coinbase is that you can put money in that and then buy coins instantly from the wallet. If you try to buy directly with your bank account, the transaction can take about a week. A credit card doesn’t have this problem, but limits are usually lower on a credit card. TIP: I almost always deposit USD in my wallet as opposed to buying coins directly from Coinbase via my bank account when using Coinbase to buy (I do this on-the-go sometimes). You can also wire money if you need the funds to be in the wallet faster. On that note, I almost always then use GDAX to buy/sell coins when I’m on a desktop (then use Coinbase as my wallet and mobile app).
TIP: If you don’t understand the tax implications of trading cryptocurrency tread very carefully. There are some nasty traps you could fall into when trading coins. For one, they are not necessarily considered “like-kind assets.” If that is confusing, then consider sticking with trading USD for coins in Coinbase until you grasp the concept. Learn about cryptocurrency and taxes.
Never put all your eggs in one basket. Diversify. While the potential to earn more is increased with the amount of money you invest into a coin, the potential to lose more is also magnified. Another way to think about it is to look at the cryptocurrency market as a whole; if you believe that this is just the beginning, then more than likely the entire market cap of cryptocurrencies will increase. What are the chances that this market cap increase will be entirely driven by one coin vs. being driven by many coins? The best way to safely capture the overall growth of cryptocurrency is to diversify and reap the benefits of growth from multiple coins. Also, fun fact — Between January 2016 and January 2018, Corgicoin has increased by 60,000x, and Verge has increased by 13,000x. During the same period, Bitcoin has increased by 34x. While you would have gotten impressive gains from Bitcoin, expanding into other coins could have landed you potentially larger ones.