They need substrate, most slings will want to burrow or at least (in the case of your aureostriata) dig a little shallow scrape to sit in. A sling without substrate will probably feel exposed and unsafe and stress out. Plus soft dirt is much better for their toes, and during molts, than hard plastic.
Well depends on if it's arboreal or terrestrial. If it's terrestrial, you should put substrate because the vast majority of sling Ts will burrow. It's their safe spot. I would still put substrate in for arboreals, but it isn't completely necessary, but recommended. For one, it helps protect against damage if it were to fall, and two it holds humidity and moisture better.