Do you think there a real difference between a plug-in and a helper application?"

Almost any program can be configured to act as a helper application for the browser. Examples of helper applications include Telnet and Excel.

The browser spawns a separate process which starts the helper application. An advantage of helper applications over plug-ins is multitasking between a helper application and the browser window. If the browser is closed down, the helper application lives on.

The browser has no control over the behavior of the helper application. The browser only has the ability to start the helper application and display the appropriate file.

A plug-in is an application program invoked by the browser. It is a dynamic code module designed to extend the capabilities of the browser by integrating a third party application program into the browser. Thus, a plug-in is part of the browser binary tree and runs inside the browser window. It cannot live on its own. Examples of plug-ins include RealAudio and Shockwave.

Helper applications are not as popular as they once were due to the fact that developers prefer to create plug-ins, ActiveX controls, and Java applets instead.

"What are the reasons for their development?
Is it a good thing to allow companies other than the manufacturer to extend your browser software?"

The existence of plug-ins, makes it possible for browser developers to focus their efforts on basic Web capabilities, while leaving the production of extended capabilities to smaller niche-oriented developers. The process works because browser developers allow their source code to remain open (freely available to developers or the general public). As a result, the plug-in developers (and anyone else, for that matter) can study each browser's technical specifications and devise compatible complementary products.

Extended capabilities are needed to implement immense amount of application-specific behaviour which would be impossible to predict during Web browser development.