Here's why Shawn (James Roday) really needed Gus' help to solve crimes on Psych. In the USA series, best friends Shawn Spencer and Burton "Gus" Guster (Dulé Hill) work for the Santa Barbara Police Department as independent consultants who assist in homicide investigations. They're able to do this because the two are partners in a psychic private detective agency called Psych, in which Shawn is the "psychic" of the duo.
Through all eight seasons of Psych, Shawn uses his so-called psychic ability to help the police get to the bottom of various murder cases on a weekly basis. While SBPD detectives Carlton Lassiter (Timothy Omundson) and Juliet O'Hara (Maggie Lawson) investigate suspects, Shawn and Gus follow their own leads to get answers. Shawn's unorthodox approach involves faking visions, communicating with inanimate objects, and more. And for most of the series, only Gus and Henry (Corbin Bernsen) are aware that Shawn's psychic antics are just a ruse. His real gift is his natural ability to observe hard-to-spot details that go unnoticed by everyone else.
Considering that neither character is a real detective, and that only Shawn is gifted with hyper-observance, the reason why Gus is his partner (apart from being his best friend) may not be all that obvious. However, Shawn did state quite simply why he needs Gus around in the Psych pilot episode. Shawn said to Gus: "I need you, because you know how I zone out when people talk." According to Shawn, Gus needs to work with him so that he can take notes and, most importantly, keep Shawn focused. It may seem like a minor detail, but Gus keeping Shawn grounded is what allows them to not stray too far.
There's no question that maintaining his focus is an issue for Shawn in Psych. Then again, it's a bit of a problem for both of them. A big part of the show's comedy comes from Shawn's tendency to get distracted by a pop culture reference, which can lead to an argument about movies with Gus right in the middle of an important investigation. But in the end, they always come to the right conclusion (even if means a few false accusations along the way). It's all part of Shawn's process, which isn't as easy glancing over a crime scene and coming up with all the clues at once. Talking with Gus and getting his input often leads to a major breakthrough. Furthermore, Gus' support and their deep friendship is integral to Shawn's success in solving cases.
It's important to note that Gus does more than just help Shawn uncover a mystery; he has a few things that he brings to the table as well. Gus, thanks to his job as a pharmaceutical salesman, has extensive knowledge of drugs and medicine, and this comes in handy a lot, considering that drugs often pop up in their investigations. Even Lassiter has discovered that Gus is useful to have around in this respect. There's also Gus' almost superhuman sense of smell, which has earned him the nickname of "The Super Sniffer". Gus' uncanny ability to identify hundreds of different scents has been an asset to the duo on multiple occasions. So though Gus may not be a real detective, there's more than one reason why Gus is irreplaceable as Shawn's partner in Psych.