This is going into advanced dynamics topics, but I will try my best to give you some valuable ideas about it.
As you might be aware there are a few different dimensions that make up the overall dynamic footprint. Simplified you can think of the overall dynamics as the product of short term and long term dynamics.
You can use expanders to improve both the long and short term dynamics. The key is to be aware of what you are doing relative to these and why.
You have mainly three scopes of application for dynamics - individual track, group, mix. So already here we have 6 dynamic dimensions where expanders can be used to improve the quality of the overall dynamics.
Expanders are then used in several ways for various reasons relating to these 6 dynamic dimensions, therefore expanders are really important when discussing advanced work with improving the dynamics of a mix.
I am not going go through them all, however one key use for expanders is to control the tendency for unwanted harsh sound characteristics (noise reduction) in the short term dynamics and to control fatigue in the long term dynamics (noise reduction). Both of these qualities are quite critical when you aim for giving the mix that little extra push up on the charts. How you set up the expanders for this is advanced.
Another type of application for expanders, in the long term dynamics, is to provide elements in the mix additional room for the signal and hence avoid having to compress as much. This is good for instance to give drums enough room in the choruses.
We are now discussing work with dynamics above pro level.