As most of these inexpensive tools tend to omit the ground altogether (such as in soldering irons), why would ground be added as a clip instead of a third prong?
Is there an advantage for it to be implemented this way?
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Some of reasons I can think of:
You don't necessarily want to ground directly to "earth". For example, you may an inline 1MOhm resistor to protect the user from electrocution (e.g. touching mains).
If you were using the earth terminal on the plug, you would not put a 1M resistor for avoiding confusion as to the purpose of the terminal - safety connection vs ESD protection connection.
You may not want to connect to mains earth at all. All you need is an equipotential working area - and that equipotential level doesn't necessarily need to be mains earth.
You could for example simply connect yourself (via wristband) and the solder sucker to an ESD mat. This would ensure you, your workspace, and your tool are at the same voltage as whatever device is in the workspace.