There are several uses for washi tape, which is why it is so popular in both crafting and planning. Here are the ways where I, personally, use washi tape in my life, in no particular order.
Decoration for Journal
A quick way to jazz up a journal page is to put some washi tape on it before you start journaling. There are several ways to do this and the limit is your imagination!
Another common use for washi tape is to make partitions. In this example, I used very thin strips of washi tape to section off areas in my Hobonichi to make them easy to spot.
Another quick way to correct mistakes in writing or journaling is to slap on some washi tape on the offending area, then write over it. For this particular case, I made use of paper tape to make it easier to write on. Regular washi tape would probably require more robust writing instruments such as sharpies or markers.
Attaching Photos and Ephemera
Washi tape is also commonly used to attach photos and ephemera to a page, whether temporarily or permanently. The ability to attach things temporarily is great in travel journaling, or even just a quick way to anchor things like receipts, bus tickets, or other random everyday ephemera.
Washi tape is also a great way to mark your things for quick identification. This is great in cases where you will be sharing your things with a group and you want a quick way of identifying which ones are yours.
And finally, I also use washi tape for gift wrapping. I was able to consume a roll or two of washi tape over the holidays as I made them the focal point of my gift wrap.