Resistance to ground for ESD protection
It's what you suspected...

Some soldering irons have the mains cable going straight into them and are safety earthed because they have exposed metal. They're not as ubiquitous as they once were, to say the least. A secondary benefit of the earthing is that it prevents static build-up.

Many modern soldering irons are part of a soldering station, having a mains powered box which delivers safety-isolated low-voltage DC to the iron itself. They don't need a safety earth on the metal body but it's earthed to prevent static build-up.

Earthing straps and so on are earthed through high resistance, for three reasons. Firstly, it stops electric shocks to the user being more hazardous because of the easy path to earth. Secondly, to reduce the discomfort or pain of a sudden discharge when a static charge is touched. Thirdly, to try and reduce the damage to components by lessening the rate of discharge through/across them when a charged part is touched.