One of my favorite multi purpose mediums in my stash is Matte Medium. It comes in both gel-like and fluid. i use them a lot in all kinds of art projects. Mostly i reach for the fluid as it usually sits on my desk ready for daily use.
I use the fluid to collage papers onto a background. I tend to paint over these collage pieces with liquid mediums. With the fluid matte medium, i can get an even layer of adhesive ensuring the paper won't bubble as if i just use something like a dot roller. I will also cover the top of the collage papers with fluid matte medium so that my paints/inks don't seep into the paper. It creates a barrier so the paper doesn't get too wet.
It dries clear so it's great to use as a glue with transarent items like this tissue paper. Again i can go over the top of this tissue paper, especially if i want to add another layer.
Matte Medium can be used to create a clear ground instead of gesso. It has tooth to grab the paint or spray ink without losing the image background like this atlas page.
i often use it as an adhesive for my vintage thread. I spread the fluid medium on the page and place the thread into it. Did you notice it in the "Growing" video?
If my journal pages are full of glossy paint like the one above, i will coat it fluid matte medium to keep the pages froms sticking together. Nothing is worse then creating beautiful pages only to have it stick to the other page & ruin both of them. This helps prevent it.