Cryptocurrencies are blockchain based. Blockchains are ongoing and public ledgers of events which record every single transaction that occurs over the cryptocurrency network.
The effect of this level of public information sharing is that any time you engage in a transaction (i.e. when you send or receive Bitcoin), the other party knows your wallet address and is therefore able to view all of your financial history, including the balance in your wallet. The other party will also be able to monitor your future transactions and can share your wallet address with others, including cybercriminals.
In order to avoid this and keep your personal anonymity and the anonymity of your cryptocurrency funds, you need to use Bitcoin tumbler to break the link between your old and new wallet address.