Diverters are not ESSENTIAL to collect rainwater off your roof. However, they are EFFIECIENT and CONVENIENT. Without a diverter you will need to REMOVE the lower portion of you downspout away and attach an the  elbow joint that you often see at the bottom of the downspouts. During the winter months, when you winterize your barrel,  you will not be collecting water.  In this case you will have water spilling out of the downspout at about 4′ above ground. This may not be great for the foundation area as over time it the pressure creates a drill effect into the surface where the water lands.  A diverter provides an easy solution to this, the lower portion of the downspout remains intact,  when you are not collecting water you close the diverter to restore the downspout. The diverter also slows the water down  as it drain on a flat surface out the tongue onto the lid, this will minimize splashing,  and directs water onto the lid, without the diverter the water will splash onto the top of the lid and there will also be more spillage.  Our diverter does not work in every application, for example,  if you cannot place the barrel in front of the downspout, the tongue will not reach the barrel. In this case you may want to use and accordion adapter for your downspout