Do I download app on both PC and smartphone?

I have both a PC and smartphone using the xfinity internet, with user name and network password already set up.  I am confused as to the downloading of the xFi app and My Account app with buttons shown on the instruction video e-mailed to me.  Do I download on both PC and smartphone, just one or neither?  Both PC and smartphone are currently using this internet with the old modem.  So, please let me know what, if anything, I need to download.