your initial response made it sound like you haven’t. It’s not UK immigration. “I read somewhere…” also smacks of hearsay rather than actual knowledge.
I applied on my own the first time and our application was denied. What they ask for is the absolute bare minimum, and they are looking for a reason to deny the visa, no matter how small. There’s no appeal and once you have been refused once they look even harder at subsequent applications.
As already stated all the agents do is what you can do yourself, but they know exactly how much information and in what format to present it.
I used an agent three times after the initial failed application and they got me the visa without any issues every time, so in my experience and opinion it was worth using them.
Just to add, the visa applications are processed in India and there is no “interview” as such. Your girlfriend/wife has to go in to submit her application alone and they just check through the paperwork and send it on to India, so there’s no chance to explain or elaborate on anything, it is purely down to whatever evidence is submitted.