@chillyboy
Most of these games are between 30 and 40 years old. Playing them on a HDTV highlights how low-definition/pixelated they truly are. To alleviate that for those who don't like the pixel aesthetic, filters have been created to smooth things out in different ways depending on the filter being used. The smoothing example given above is a popular approach and not very processor intensive, but some people find it too blurry for their tastes.
There are also filters like this that try to take a middle ground by smoothing the pixelation, while sharpening the resulting edge, but some people find this approach too unnatural or plasticy. Something that seems to satisfy purists and revisionists alike is to recreate a CRT look on your LCD monitor like this. Personally, I take the path of least resistance and leave the games in their unfiltered pixelated glory, but at the end of the day, it's all about personal preference. The question you have to ask is, which one of the many compromises is least objectionable to yourself.