Privacy is the prevention of unauthorized extraction of information from communications over an insecure channel, it is the best-known problem that field of cryptography attempts to solve.
There are many reasons why people need privacy: personal, psychological, financial, social, etc. People encrypt for the same reason they close and lock their doors. Encryption can protect criminals, but so do locked doors.
Although there is no explicit "right of privacy" enumerated in the U.S. Constitution, the assumption that an individual is to be secure from absent a valid warrant is central, and dates back to the Carta Magna. That includes random searches and invasive tactics to catch criminals.
There has never been a ruling or law that persons have to speak in a language understandable by wiretappers. Similarly, there hasn’t been a rule banning encrypted communication.
Privacy is essentially tied to fungibility in cryptocurrencies. As cryptocurrencies with a public ledger such as Bitcoin often lose fungibility, as some coins are treated as more acceptable than others. A user can unknowingly accept coins involved in questionable transactions, which can taint the entire amount of his/her wallet and result in a loss of reputation.
Beam achieves privacy by default, unlike other cryptosystems which require an opt-in. Zcash or Monero, for instance, requires users to choose a private transaction, an option which significantly slows down the transaction, and consequently is only used by 4% of the network, in the case of Zcash.