They are portable (i.e. Mobile) sites that can be deployed to help with Communications coverage for large events or situations. They can be set up anywhere in the state and work as an extra site on the MARCS system. For example, one of the sites was used for the 2016 Republican Convention in Cleveland. They're not used often, but there are two available if needed.
And just to clarify, the TOW sites ("Tower On Wheels")
can be connected to and operate as another site on the main system to fill in for a site which has been damaged and is undergoing repair, but are usually operated stand-alone with no connection to the main system.
There are three total; two share a system ID, which means if all three are deployed at the same time the two which share a system ID would have to be deployed pretty far apart to prevent interference.