Giving a person their own outside number without buying them their own phone line. It is older than VoIP and it is why extensions work.
Historically a company had a few lines and one switchboard number, and a human operator transferred you. DID let the phone network deliver a call straight to an extension by giving that extension its own dialable number, sharing the underlying lines.
Everything about modern business telephony still works this way — the hardware just became software.
It is what lets you give the accounts team a number that is not the main line, run a campaign on its own number so you can measure it, or hand a big customer a number that always reaches their account manager.
A DID is not a separate phone system and does not multiply your seat cost. It is a number pointed at a route you already have.
If a vendor is charging you per seat again for a second number, that is a pricing decision on their part, not a technical necessity.
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