The usual reasons for this type of plastic barrier is compliance with electrical safety regulations. During servicing, boilers often need to be live while the cover is off, which means there needs to be some protection for the winding from being accidentally touched by a finder or poked with a probe.
There's a lot of distance in terms of safety and testing between the death-boxes Professor Brainstorm and his elves build from catalogue parts in their physics laboratories and what can be sold to the general public. I've known some very smart people do some very dangerous things because they mistakeningly thought that being an 'academic' meant that safety legislation didn't apply to them. The HSE doesn't share this opinion.