Having a licensed Simply Modbus Master with an exclusive license key offers several benefits:

The free demo is fully functional but limits you to 6 reads/writes per session. For quick pings or checking a slave ID, the demo is usually all you need. Support the Dev:

A Simply Modbus license is not an automated digital download; it is a human-verified purchase. Manual Fulfillment

The developer sells individual licenses. Any key shared publicly online (on forums, YouTube, torrent sites, etc.) is almost certainly either:

: A portable license that allows you to use the software on any computer by plugging in a registered USB drive. Modbus TCP/RTU simulation using free, open-source alternatives like QModMaster