All I can speak to is what was observed when the Comcast tech put in Internet access. Step 1 was to move a wire in the junction box on the street. (We are all underground.) Then he connected to the existing splitter where the cable meets the house. We already had cable TV. Result? That plus a cable modem gave me great Internet. But - we found the TV badly degraded. Comcast sent out a different tech - he looked at the splitter and shook his head. "Wrong splitter!" he said.
He changed the splitter and both TV and Internet were great.
Now, phase 3 - this year I added Comcast Digital Voice to replace my Qwest landline. Same process at the street. The junction can was opened, a couple of wires were moved. Can closed and resealed.
In the house, a new modem (triple function Arris) was put in and the tech communicated with the office and cleared the system. All has worked perfect ever since.
Bottom line - different providers and areas impose different requirements. I found the different splitter to be a key element.