The baffles in your tank serve 2 main purposes: to give structure to the tank, and to control the movement of the liquid load while driving.
Properly engineered baffles in good condition are critical to good structure of the tank. Your vacuum truck tank should be able to withstand full vacuum. In order to do that, the baffles (and other tank components such as internal or external rings) give structural support to the vacuum tank shell. A well-engineered vacuum tank will have proper baffle size, spacing, and cut-outs in them to accomplish this. An older tank with rusted or damaged baffles runs the risk of the tank collapsing under vacuum.
A liquid waste load moves a lot while driving. A 4000-gallon tank with a partial load could have 20-30,000 pounds of waste moving around. A quick stop and all that weight comes flying to the front to push your truck forward. Properly placed baffles will greatly minimize that movement, while still allowing waste to flow out the bottom to drain the tank.
It is important to have a vacuum tank that is properly engineered, properly built, and properly maintained.
The action item here for any vacuum truck owner is to do regular tank inspections, including inspecting the condition of your baffles. Look for any corrosion that might impact the integrity of the baffle, inspect the welds, and inspect the general condition. See HERE for a video on things to inspect on your tank regularly. Repair or replace any damaged portion immediately.