It improves readability. One reads shell commands from left to right, following the logic of each step in the pipeline individually. It makes sense to start with a file, then pipe that to command 1, pipe that to command 2, etc. It makes less sense to start with command 1 into which we put the file and then take the output of command 1 from before the file and put that into command 2 etc.
It also improves future editability. If you wanted to change the input to command 1, you go tweak the bit to the left of command 1 instead of having to hunt down where the < is stuck in the middle somewhere and potentially remove it and replace it with something off to the left anyway depending on whether it's a file or the output of some other command.
Using cat as the first step into a pipeline is almost always the right thing to do!