#3. Goldberg is a big and marketable name for SummerSlam
The most obvious reason why Goldberg returned to RAW is that the company needs a big marketable name for SummerSlam. With the event happening at the Allegiant Stadium, it's set to host over 60,000 people and will be a bigger show than WrestleMania this year.
WrestleMania 37 lacked part-time superstars, but they weren't needed as much for a show with a little over 25,000 fans on each night. There are no restrictions this time around and WWE needs to sell tickets. John Cena certainly sells tickets and so does Goldberg.
As much as the vocal WWE fanbase resents him, there is a reality that must be accepted - he puts eyes on the product and is a legitimate draw who can sell tickets. With WWE's current system, there are no larger-than-life stars this generation, and the chances of there ever being a Goldberg-level star seem slim to none.
WWE doesn't want to rely on superstars to draw, but while they have the option of names like Goldberg and John Cena, they won't hesitate to take it.