Now let’s be clear about a few things up front. First off, you don’t neeeeeed washi tape. Any regular tape will do for most normal uses, and will likely do a better job at actually sticking things together. Secondly, you can write on many tapes, both standard and of the washi variety, with ballpoint pens, permanent markers, and pencils. And third, they can be expensive, especially when compared to the basics.
So, why washi tape? For the same reason you buy that purple gel ink pen, or that shimmer ink, or that eleventh notebook. They are fun, and allow you to make your stationery personal. What you put on the page is an expression of yourself, and washi tape fits right in with all of it.