MySql Next-Key Lock Behavior

[Bill Wen]

Regarding the official document version 8.4. Why Next-key behaves like this? "a next-key lock is an index-record lock plus a gap lock on the gap preceding the index record. " For example, I have index on id, query select * from table where id > 3 for update. Other sessions can't insert id = 3. I understand that (2, 3] is the gap preceding the index record but I don't understand why this record need to be locked. Please explain this.

Bill