I don't use Washi tape as part of my day to day planning but I do use it to disguise recycled Filofax pages or as and aid to making my own pages. Find them here and here. My latest buy was £1.28 for a pack of rainbow colours from Japan.
The box was rather squashed and it wasn't easy to get out each colour so I cut a side out and then stuck the other sides together with - wait for it - Washi!
Now I have a bright an colourful carrier and the tapes don't get lost.