"so helpful that" is correct and usable in written English.
You can use it when you want to express that someone or something is helpful to an extreme degree. For example: "The volunteers were so helpful that I didn't have to lift a finger during the event."
And they may have been so helpful that they gave modern humans an edge.
Not only that, the staff are so helpful that it makes book hunting a real pleasure.
(He found the instruction so helpful that he eventually joined the faculty himself and years later went on the board).
The meters are so helpful that some riders are now calling for their use to be restricted in competition.
But Judge Moro determined that his assistance had been so helpful that he was given the minimum sentence allowed in his plea agreement.
Malnourished children are prone to infections, and the drugs — either amoxicillin or cefdinir — were so helpful that researchers said medical practice should change immediately to include an antibiotic in the routine treatment of severe malnutrition.
"For me, the web was so helpful to that process.
Many customers discovered Hubspot because their content is so darned helpful that people share it widely.
Further monitoring over 17 years--which amounted to field data on nesting, molting, and responses to threats of predation--showed that males dirtied themselves immediately after their mates had laid eggs--just after they no longer needed the bright plumage that was so helpful in winning that mate.
Nor was it surprising that all those Bloomie's shoppers could be so helpful, since by that time Valium, which had been introduced in 1963, was the best-selling prescription drug in America, with billions of blue or yellow or white pills, each stamped with a trademark V, sold every year.
It is possible that the reason they found this support so helpful was that they were in a life stage when the continued help of their parents could ease their adjustment into adulthood.
