Oh, they do... but my point is that they are not any more knowledgable than investigators with any larger agency (and several smaller ones).
They just have a history that they like to toot the horn of. Todays Rangers are nothing like the Rangers of the old days.
Many of the larger state agencies have their own OIG officers that are just as qualified and trained as a Texas Ranger, and in some cases even more so.
You have to look at where they are drawn from... the DPS officers rarely enforce criminal law unless it deals with traffic or drugs. Their investigative skills outside that limited zone are lacking. In fact, many of them are not even up to date on the majority of criminal laws.
That is not to take away from them, as they are primarily charged with traffic enforcement and drug interdiction. In my 15 years of patrol, I can count on one hand the number of times I know of them investigating something like a robbery. They would wait until the Sheriff Department or local PD showed up to investigate that offense.
Now, take an investigator with a larger PD, generally they have come out of patrol, which investigates criminal and traffic offenses, so they have a more rounded exposure to enforcement and investigations.