Hello Lawrence, and welcome! You're right, anyone with access to the board will be able to edit all the cards within that board, even if they're not a card member.
The main reason for making someone a card member is so they'll receive notifications when that card is modified or a comment is added. This can be useful if they're the person responsible for the task the card represents, or if they just want to follow along and receive updates.
People often use card membership to represent assignment. But it's up to you how you'd like to use it!
There's also an option to Watch a card without joining it, if you want to get notifications but don't want to be a card member: https://support.atlassian.com/trello/docs/watching-cards-lists-and-boards/
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