Bitcoin is also a currency. I think you can say it is a tool because we can store money in here.
The total supply of Bitcoin is 21 million and it has historically held its value for the long term. This makes it relatively easy to send remittances and Bitcoin can be seen as a "digital savings account".
Its small supply compared to others is one of the reason on why its value can rise. Its high value is also what makes BTC popular, so like you said, it is now easy for us to send and receive money. But apart from it, its decentralization aspect is also one of the factor to make it possible.
By putting aside some regular portion, just like fixed deposits used to be. It is a type of investment or opportunity to make a profit. "Buy low, sell high" is an asset that you hold for the long term, not something that you "buy when the market is up and sell when it's low".
I thought we are talking about savings here? But yeah, it can act as investment opportunity as well. Buy low sell high is only one of the methods for investing but BTC is a type of asset that we can as well hold for the long-term. Buying high is also possible and it is called as DCA. Some can sell at low but it does not mean that they are now a loser.