I am a former member of the Maryland Defense Force so while I don't know much about the Texas State Guard I do know about the State Guard concept.
It is the official state militia, brought back by executive order of Ronald Reagan some years back.
They are a civil defense organization that is part of the military department of your state (like the National Guard). Duties and benefits (if any) vary a little from state to state. At minimum you'll be backup for the National Guard, should something be big enough that they aren't enough or should a substantial number of them be on Federal duty you can be called upon for the same kind of state duty they can be called for. Some states only use them as a kind of reserve for the guard while others will actually utilize the state guard (I remember when I was in the MDF a decade ago there was quite a buzz when CA called up their state guard to fight forest fires). In MD we "prepared" in case of nuclear war and other "worst case scenarios", and being called on for riot control or traffic control, but mainly our job was going to be to be a laison between the Guard and the families if there was a large scale call up of the National Guard and we were to make sure the Armories were maintained and safe.
Mostly it was made up of old vets (mostly Vietnam vets) and young guys who were making up their minds about whether or not they wanted to join the military. There were a few non-wartime veterans and a few who had never served but not many (my unit was me a then 20yo who had recently been discharged from the Army for a bad ankle, a former Marine officer in Vietnam, a former Marine 1SGT in Vietnam, a 20 something school teacher, a 50yo housewife, a 40yo NSA analyst and later a late 30s former Marine officer).