There are tons of applications in wet/dry mix. My English is not that good, so I can't explain it with all the subtilities, but it's all about envelope and tone. It's another trick to get different results that you would not obtain using only the output of a compressor. Some great compressors don't have side chain and the result is a pumping mix when you go too deep with the threshold. Some other compressors are eating the bottom end. For both issues, wet/dry is a cool solution...for me.