Why do turbofan engines have low pressure compressors? In my understanding turbofan engines usually have 2 stages of compressors:    


one low pressure compressor    
followed by a high pressure compressor


But it looks like the high pressure compressor cancels the job done by the low pressure compressor, doesn't it?

What's the point of decreasing pressure in the first compressor if you then increase it in the second compressor?  Why not using only the high pressure compressor?
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.