why does wheel generate egg-info folder?

I am using wheel to build my package using this command which is supposed to generate built distribution package:

python setup.py bdist_wheel

It indeed generated a dist/packagename.whl under root folder but meanwhile it also generated packagename.egg-info under src folder, as show below:

package
   | dist
      packagename.py3-none-any.whl
   | src
      | packagename.egg-info
      | codes
   setup.py

So my question is why packagename.egg-info is generated under src?