SCNR. Seriously though, molten salts are great heat transfer media, a bit like water, just at higher temperature and still low pressure. They are also completely immune to radiation damage, which is a big deal if you have 14MeV neutrons flying around and something has to stop them. Chemistry in molten salts is also interesting. Things can be extracted (say, into a molten metal phase) or precipitated, and some reactions just happen at 600 degrees while they need platinum and palladium catalysts at room temperature. What's not to love?

In a molten salt reactor, there is very little volatile material inside the containment building. The salt itself does not have high vapor pressure, even in accident conditions. As a result, the size (and cost) of the containment building can be radically reduced. Moltex's design, for example, reduces the cost (per unit of power output) of the containment building by a factor of 5 vs. LWRs.