The THW is organised similar to an army and has equipment to restore infrastructure in the event of a catastrophe (natural or man-made) and to build temporary protections if required. Similar to pioneers they can provide quick temporal solutions for power, communications (including telecoms and bridges) and water. They have a very heavy focus on technical solutions and can only provide basic medical assistance. The idea is that the THW restores access to cut-off areas for emergency services and provides protection to prevent the situation deteriorating. Other services use this infrastructure to provide medical and light technical assistance, etc.

I am struggling to find references at the moment, but from what I remember, due to the history of putsches etc, in Germany the army could for a long time not be deployed internally. So while in other countries the actual army pioneers would be called in to clear roads after a flood, this was not possible in Germany. So the solution was to build a civil version of this, which as a side-effect was actually completely independent of the armed forces. Since the army cannot use these capabilities offensively, this structure helps calming twitchy neighbours after starting two world wars in a row. It is one of many examples of decentralisation of powers and capabilities in post WWII Germany.

The lines between different volunteer forces and structures are probably converging globally and most countries have similar capabilities available within their volunteers. Looking up the SES, it seems to be doing a lot of things including heavy lifting and rescuing people from cars crashes. The very specialised THW would only do the first, and (volunteer?) fire services do the second. If that is worth the distinction (and this post), I dont know. But it probably leads to the THW volunteers having much pride in being called in when the equipment of the fire service is not good enough, while seeing very little action and have therefore a lot of time to support the local brewer.