I personally always run an internal my current 1 being the F4 also along with my externals. My reasons well if I need to use any chemical filtration its a quick fix. I also use a ventura to boost the surface movement. Great also if I need an extra matured filter for say a hospital tank in an emergency as I dont have one running or less I need it, thinking of environment here

. I also find them useful if you want to try anything new like I did with some mysterious wonder products such as algone etc.
Obviously it acts as an extra mechanical and biological filter also.