I suppose you could say the growth of the internet has seen forums on all sort of subjects develop, immigration included but even before the internet there would always be people tackling some things themselves and most governement departments have always had some sort of explanatory notes to go with application forms, immigration no different in that regard.

Now there's possibly even more information online from the immigration department and on associated sites like for state nomination, in some cases it proving to be information overload for some people.

It is cases like that and more where there are more complex issues that agents can be of value.
Similarly if you're not the type of person who wants to commit to some effort in making sure you have an accurate and well presented application a good agent can again be of value.
And obviously there can even be exceptions with agents for some may not be so good and in using an agent you do lose some control.
Using an agent will also not lessen the work you need to do in collating all the relevant documents to go along with an application and other than if an agent prevents mistakes being made, an application will not be processed quicker.

The type of visa you wish to apply for can also affect complexity and there may even be different options you want to look at, a good agent being prepared to discuss alternatives.

For anyone considering whether to use an agent or not, I'd suggest that first they have a good look at the Immi site to see how they go getting an idea of which visa might suit them and understand something of the process.
Then at least if they decide an agent is for them, they'll also have some idea of what they'll still need to do and what questions to be asking of an agent during the processing.
Workers - Visas & Immigration is a reasonable page to start from and if going to the Professionals area, you'll find a What's New link at top of the next page, and always good for a read.

I've heard of Visa Bureau but not in detail re anyone using them but suspect just as there are many immigration agents in business there're also quite a few organisations with similar names.

I'd recommend to anyone considering an agent they use a MARA registered agent, there being details on the Immi site re that.