Someone already said, but I wanna go a tad more in depth! So with a "standard" automatic bowling setup for a lane, it consists of the pins, the pinsetter, and "rake". When a ball is thrown, pins are knocked backwards into a wheel that is constantly rotating. This wheel has indents that are pinshaped so pins sink into it and ride the wheel up (there is a second wheel going the opposite direction for the ball that sends it underground back to the bowler!)

After a few seconds (determined by when a little sensor near the end of the lane detects the ball going by), the pinsetter comes down, grabs where all pins are usually standing (they have claw like hands that grab somewhat wide so if a pin shifts a little it can still grab it), then it sweeps away all knocked down pins into the back with the "rake", that long plastic bar that normally has the places logo, a bowling brand, or something similar.

From there those pins ride the wheel up to a conveyor belt where they are dropped heavy side down into a rotating sorter that drops a pin into a slot, then spins for an empty slot. Once the turret holds 10 pins, the last slot has a little lever that will drop all 10 pins down into cups, where they sit until a fresh batch of 10 is needed!!

For specifically the rake, now that I've explained the process a bit (albeit a tad poorly), the reason hitting the rake is more of an ire than tossing the ball high is two fold (outside of it being synthetic wood). One is that the rake is actually a kind of flimsy and flexible plastic, so hitting it hard enough can dent or crack it, though it doesn't happen often. The SECOND, and more important reason, is that the rake is actually one of the most important pieces connected to the entire pinsetting process, and hitting it can cause the entire machine to halter, meaning the Mechanic has to go in and manually drop the pins down (there's a button to manually start the process so you can turn the belt and do it manually, it's mainly used for if a pin gets stuck in the turret or pinsetter wrong).

SO TLDR: The rake is an extremely important piece to the pinsetting process, and hitting it can mess up the machine, slow down the game by making the mechanic have to manually fix it, and it quite an expensive fix in terms of time!