cash are not that easy to use in different countries, you need to exchange them everytime, a pain in the ass, bitcoin is a global currency, and very easy too move also
and cannot be faked like cash so easily, add on top of this, the less fees required...
Yes bitcoin is can be used all over the world because as like what you said it was a global currency and easy to transfer to other wallet. Unlike cash, every country have they own type of cash dollar, philippine money, and so on and so forth. So if you go to other country you should convert your money first to their fiat so that you can used it. In addition, bitcoin was more easy to earn than cash.
Yes local currencies are hard to use if you are crossing borders, every time you buy you have to exchange your money to their money. It's not so satisfying when you are new to their country because you just have to go to somewhere to exchange your money, and to add that their taxation fees is killing your money slowly.
Bitcoin is like your wallet all over the world, but for now it's not that legal to some other countries. So it's just limited to use in some areas but it will soon be known as a universal currency.