<li>Good compatiblity with the original libraries, I think.</li>
<li>You can force the resolution and even MSAA for both Glide and DirectX which can really improve the appearance of games with hardwired low-resolution support.</li>
<li>Support for hw/driver compatibility with some really old videocards like GF4 Ti4800 and ATI Radeon 8500 is also available for rare games needing that.</li>
<li>You can force windowed appearance and simple color-management and dithering (retro feeling) as postprocess effects.</li>
<li>For really old games running at their original low resolution you can upscale the image with various resampling filters like bicubic and lanczos. Or just apply simple pixel multiplication.</li>