own digital currency

The same functionality like any other currency, to transfer value between people. What you might wonder is why would Bitcoin be better then other currency? Well, the answer is absolutely everything. It is faster to send bitcoins across the world then any other currency. It is cheaper by design to send it then any other currency as well, as it removes the costly and slow process of validation of a transfer in a centralized system, here it is done by cryptography. It deosn't have inflation, not even a government can take bitcoins from you nor can they freeze your account. It takes a fraction of a second to create a Bitcoin address (something like a bank account, but obviously decentralized and only based on mathematics). It requires no personal information to use it, so you can be anonymous. It is more secure then other transfers, it is smarter as it offers many more features like sending coins to a multisig address (for example you can define that any 2 out of 3 addresses can withdraw the funds you sent). And all of this in a decentralized way, where you need to trust no website, no bank, no government, no company, just mathematics. It is like an invention of the Internet in a financial industry. Sounds good?

You already knew the functionality of Bitcoin, what you wondered is why is it more useful then fiat currency, as it makes sense that people wouldn't switch if they didn't have a good reason to. So that is an answer to your second question, why would anyone use your cryptocurrency, how is it more useful then Bitcoin? There are many cryptos that offer many fancy features, big companies are behind them, but still, no one ever took the crown of Bitcoin and probably never will. The improvement, is just not big enough to motivate people to switch from the oldest and safest cryptocurrency there ever was.

You can make an altcoin as easy as copying the code of Bitcoin Core client from Github and making the smallest of protocol changes to make it different so that it isn't a client for Bitcoin, but for another coin that is just like Bitcoin. Many did this by the way, many even added improvements, but it got nowhere.

My advice, don't spend time learning how to create something before you know why you are creating it.