Why should I use Bitcoin? I already have a bank account/credit card/Paypal account.?

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This ultimately boils down into "Why should we use cryptocurrencies when fiat currencies already exist?"

Having full control of our money

Most people saves their money on banks or some kind of bank (like paypal you mentioned). This was the most secure way to store our monetary value before blockchains come around. Unfortunately, banks need to make like any businesses. They gamble money we save on them all the time by lending them to other people who may or may not be able to pay back. Banks even gamble with money they don't have! (https://www.investopedia.com/terms/f/fractionalreservebanking.asp) When they play with our money, what's stopping them from using the money to invest or fund shady things we may not agree with?

Banks  theoretically are able to confiscate your money/asset saved on them anytime they wish whether they goes under by bankruptcy or simply being mean. For example there has been countless example Paypal has denied their user from the funds they own themselves. Banks may also refuse to do transactions for you with your own money for whatever reason.

The only alternative we had was to hold our own money in cash but obviously it's not convenient when we need to pay someone we can't meet physically. Now with the blockchain technology (including Bitcoin obviously) we can put our monetary value there and we know for sure that it's not being moved around by someone else. We have full control of our money in digital form that can be transferred as we wish.

Cheap transaction and fast global transaction settlement time

Sending funds to other countries is expensive if done from a bank account to another bank account directly. Fees for wire transfers averages at 25 USD for outgoing transfers and may take a few days to settle.  This is not the case with cryptocurrencies. The current average Bitcoin transaction fee at the time of writing is 43 cents and transactions settles within a few hours at worst. Bitcoin is a particularly sluggish AND expensive blockchain to do transaction, Steem for instance has NO transaction fee and the transaction settles within 3-4 seconds!

People who regularly do transfers with large amount can also take advantage of low transfer fees. Most credit/debit cards (and Paypal) charge percentage fees based on the transfer amount. Paypal for example charges 0.30 USD +2.9% of the transaction amount as fees when you do international transfer. So if you transfer 1000 USD the fee would be 29.30 USD. That's way more expensive compared to just using crytocurrencies. Almost all cryptocurrencies don't care about the transaction amount so whether it's $10 or $1 Million the fee is still around the same.

Protecting our money from the government

Almost all fiat currency has their supplies controlled by government or some kind of central banks or both while for cryptocurrencies has their supply controlled by a consensus run by a lot people so that everyone who use a particular token (like Bitcoin) can expect how much new tokens will be issued. This can change, however any change on blockchain requires consensus to be implemented while government/central banks can over-inflate their currency anytime they want and people won't be able to stop them. We can protect our own monetary value from the government by using cryptocurrencies. This is more relevant for people from third-world countries where the government is extremely corrupt and the volatility of cryptocurrencies matters less.

So...

Using cryptocurrencies also have its drawbacks compared to fiat but i won't write them here as it's not what being questioned. It's not black and white. It's better to do research if you wish to use cryptocurrencies.