Screen printing uses a thick layer of ink that is pressed through a stencil and directly on to the t-shirt. It’s kind of like dying just a portion of the t-shirt where your design is. Because of that, your design will appear slightly raised off of the surface of the t-shirt, unlike DTG, which will appear as if it were printed directly on the shirt itself.
Screen printing is best used for simpler designs that do not use a lot of color as well as being better for large batches as opposed to printing as needed, or printing a very small batch, at which DTG is better.
And while it may seem that screen printing is inferior to Direct to Garment printing, it still has its benefits, especially if you’re printing a large amount of shirts.