In ca. 1910, when Standard Arms was in business, it wasn't so much why a buyer would want one or the other - as both autoloading & slide action rifles were relatively new on the American sporting scene at that time.
IMO, Standard most likely felt that a dual-use firearm would appeal to two markets (auto AND slide) instead of just one (auto OR slide).
While there is some truth to the idea of it appealing to pump guys, the bigger concept was a backup system for the unreliable gas system. As mentioned, it was the first mass produced gas operated semi rifle, and we're talking ca 1910. The company went out of business due to the rep these rifles got. Instead of getting billed as "latest and greatest" it was more like "new technology, but unreliable, dangerous...". Logically, if the gun has a new technology, and it doesn't work consistantly, then you would need a backup system. Without the pump, some issues/failures would render the gun inoperable. This could have been esp bad in say a LE or hunting scenario (dangerous game).
They were made in .30, .32 and 35 remington. The model M that Jim K mentioned was made at the end, when they decided to pull the plug on the model G, perhaps to improve sales, or to use up already made parts, (or ?) but it ended up being too little, too late for Standard Arms. There was some 7000 Model G guns made, but only 1000 to 2000 of the model M making it one rare bird today. I read somewhere that Numrich bought out the spare parts from the failed company (or from whoever acquired the parts from the company) and assembled more guns with the parts on hand. Numbers vary on how many were assembled after the factory shutdown.
I have 2 of these myself, one of each, I will try and post pics later. They certainly have a special place in history and as Bob mentioned, not seen very often today. I have never fired either of mine.