KeepTogether Tablix Property Setting should be used with care. This can cause whole report to load in single page and your report builder or web application (Reportviewer Control) can become very very slow in rendering.

This property can be set as shown in below image:

As per MSDN link, Indicates the entire tablix (all repeated sections) should be kept together on one page if possible.

If you are rendering report in HTML, InteractiveHeight property controls soft page break and in case we are getting large number of records, this causes soft page break. By default this is set to 11 inch.

But, when we set KeepTogether on Tablix then InteraciveHeight property is ignored and SSRS renders all the records in single page. This can cause the application to become irresponsive over period of time till the rendering for all the records gets completed.

KeepTogether property also exists at group level, but it in almost every causes this will not cause harm as if we are implanting grouping, the data will be restricted. KeetTogather group property can be accessible as shown in below image:

Recently this thread has been posted on MSDN forums and this is inspiration for this post. 

In SSRS 2008 engine, enhancement was done for execution of report. Execution only happens for the requested page. By this setting, we are not able to get this benefit as we are rendering in a single page and in case of very big report, this can also impact other report's execution