The toolbar prototype was the Alexa toolbar way back in the mid 90s. I forget what it was that the Alexa bar was supposed to do for people*, but it’s only real raison d’etre was as to track it’s users’ travels around the net so that Alexa could aggregate it for sale as traffic data.
That’s still the model most follow. Some do a little more for users while others do less, but all exist primarily for the benefit of their manufacturers. The Alexa bar’s purpose was to track and transmit traffic data to the Alexa corp, while the Google bar’s job is to tell Google increasingly detailed info about users, which allows Google to make more money by selling your attention to advertisers.
In short, toolbars are never about the user’s convenience; they’re always about the manufacturer’s**. Thus, there’s little or no benefit for users. However, running any piece of software is a drain on the host computer’s overhead. They eat processor cycles and help themselves to your internet connection to phone home; they compromise your privacy. Some slow down pageloads, making the internet slower, or jump in and interrupt the tasks, searches, downloads or network activity you’re trying to accomplish.
As well, because toolbars are intended to monitor user behaviour, they’re designed to be on all the time. They’re rarely programs that a user can easily start or stop. They don’t want you to so they make it difficult. To do this reliably you have to use the task manager. Even then, many are designed to restart themselves automatically.
And finally, toolbars proliferate. Its not unusual for people to have several different toolbars all running at once, which means that the slowing-down effects get multiplied. This also contributes to instability. Not only do they interfere with programs users are trying to run, but they can also interfere with each other, causing freeze-ups and page hangs. From the IT guy’s point of view, this makes shooting trouble harder because it increases the number of programs participating in whatever operation it is that’s broken.
So the final answer is that toolbars are parasites, giving nothing to the host that’s worth the cost of their drawbacks.
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*But it was nothing that users needed; Netscape and then IE already did anything that Alexa did. Their strategy was to get the bar installed in any way they could and then hope that users get habituated to it or think the bar is necessary.
** Which is why one very common toolbar distribution strategy is to pay third party makers of popular software to include the bar as a default-on & hidden 'option' during installation. Adobe is a frequent villain; Acrobat and flash are regularly larded up with paid non-Adobe hitchhikers.