If you have files, folders, or entire drives that need to be completely protected from third parties, then yes, you need encryption.
Whether you have logins, firewalls, and anti-virus programs unless your data is encrypted someone can still find a way to access it - especially if they have physical access to the system or have breached your network security in some way.
That being said, encryption is mostly necessary to protect the privacy and confidentiality of information, including that of clients and employees, your own family, and financial data like accounting, card, and bank details.
If you’re also someone who stores passwords unsecured or who wants a way to access all of them via a master one, encryption software may be the solution.