PEM and DER are the same data, two encodings
A PEM certificate is just a DER-encoded (binary ASN.1) certificate, base64-encoded and wrapped in -----BEGIN CERTIFICATE----- / -----END CERTIFICATE----- lines so it can be handled as plain text. This tool converts directly between the two: strip the PEM wrapper and base64-decode to get DER bytes, or base64-encode DER bytes and add the wrapper to get PEM. Everything happens in your browser - your certificate is never uploaded anywhere.
Why different systems expect different formats
Some servers and tools (particularly on Windows and in Java environments) expect DER, while most open-source tools (Apache, Nginx, OpenSSL) expect PEM - this covers the conversion either direction needs, which comes up often when moving a certificate between a Windows IIS server and a Linux-based one, or between a Java keystore and an OpenSSL-based tool.
Why DER is a binary format and PEM is text
DER is the raw binary ASN.1 encoding the certificate standard actually defines - compact, but not safely transmittable through systems that expect plain text (like pasting into a config file or an email). PEM exists specifically to solve that: the same binary data, base64-encoded into text-safe characters and wrapped with clear markers, so it survives being copied, pasted, and stored as a plain text file.
Why this tool is scoped to certificates, not private keys
Private keys carry higher-stakes handling considerations than certificates (a certificate is public by design; a private key must stay secret), so converting a private key's format is better handled with a trusted local tool like OpenSSL, run on a machine you control, rather than any web-based converter.