Some custom SharePoint products make changes to the web.config file of a web application via SPWebConfigModification calls in order to support particular features. There is a web.config file for each web application on your farm and they reside on each WFE server. If your farm has 3 WFE servers and 2 web applications, you have 6 web.config files.
For SharePoint 2010, Bamboo product installs may add entries to the web.confg to support Telerik functionality. Telerik is a 3rd party that provides some user interface controls for some Bamboo products. See Bamboo products that use Telerik for a list.
For SharePoint 2007, Bamboo product installs may add entries to the web.confg to support Telerik and AJAX functionality. We use AJAX to allow web applications to send data to, and retrieve data from, a server asynchronously (in the background) without interfering with the display and behavior of the existing page. See Bamboo products that use AJAX for more information.
At times, some of these changes need to be modified (via additions to the web.config), or partially removed for particular web applications on your SharePoint farm.
It is neither recommended nor a best practice to manually update a web.config file. Manual changes are not tracked by SharePoint in the configuration database; as such, they will likely cause future problems for users on the farm. For example, when creating a new web application, the new web.config is generated automatically by SharePoint and the auto-generated file will not include any changes that were made manually.
Use PowerShell to programmatically remove or add web.config modifications so that changes are updated in the configuration database as well as on the server(s). An added benefit of making changes via PowerShell is that SharePoint will propagate the changes, so if you have 6 web.config files on your farm, you need to run the PowerShell script only once.
For more information, please see: