Different quality, support, tradeoffs, maturity levels, focuses, and add-ons?

Might as well ask "what good is competing webservers".

The same reason that it's good to have different clients for other protocols. Some clients lean heavily to one side on the CLI vs GUI vs VR debate, some choose to have convenience over technical complexity and so make choices that other Gemini users would disagree with, some allow a degree of automation, some are a multi-protocol client that has Gemini as just one supported option.

Read the spec, it's not long. It leaves plenty of room for variation.

Not keeping to the same spec is one of the reasons why modern web is a dumpster fire.