Many multi-track tape recorders have options for manipulating pitch and speed. I've found that useful in some instances, especially manipulating the pitch on pre-recorded material to create interesting modulations. You can also make drone loops where you can add/remove tones or sounds and/or place them higher or lower in the mix on the go using the volume controls. That way the tape recorder can become an instrument that can be "played". Here some bleedthrough between tracks can give some interesting effects. I've seen videos where people use reel to reel tape players to make long tape loops having the tape go around all kinds of objects, including objects that make the tape gradually selfdestruct - I've never tried anything like that, but it looks like fun and might give interesting textures. Personally, I mainly use my old Tascam to give stuff more grit by overloading its inputs and outputs or simply by recording on tape. So I'm not really doing that much tape manipulation, just using the 4-track for recording either as a pre-amp or as a recording device.
