Why do turbofan engines have low pressure compressors?
The low pressure compressor does not reduce pressure. It increases it, but operates at a lower pressure than the high pressure compressor.  Some engines also have an intermediate pressure (IP) stage.

https://en.wikipedia.org/wiki/Gas_turbine_engine_compressors

https://www.grc.nasa.gov/www/k-12/airplane/compress.html

Each stage increases the pressure as the air is forced back into the flame tubes.