Why "avoid well-known ports" matters

Ports 0-1023 are the IANA-designated "well-known" range, conventionally reserved for standard services — 22 for SSH, 80/443 for HTTP/HTTPS, 53 for DNS, and so on. On most Unix-like systems, binding to a port in this range also requires elevated privileges (root, or a capability like CAP_NET_BIND_SERVICE on Linux), which is a second, independent reason to avoid it when picking a port for a new custom service: even if the number happens to be free, using it forces the service to run with more privilege than it otherwise would need.

The 1024-65535 range (registered and dynamic/private ports combined) is where new services conventionally pick a number, and it's what this tool defaults to avoiding the well-known range for.