What’s the deal then? Well, when we played video games on CRT televisions, there would be faint horizontal lines between the rows of pixels. Kinda didn’t notice back in the day, but when you play an old Mega Drive through a modern HDTV, you will instantly see something is missing.
Modern TVs have built in picture scalers, so a 320×240 picture gets blown up to fit your gloriously large display. You’ll notice the pixels look big and slightly smudgy, edges bleed and the picture just looks a little… smoothed. All arcade games used to have scanlines. I swear a scanline generator/emphasiser was built into the Hanatrex monitors! Adding scanlines makes an arcade game feel even more authentic. I still play all my retro games on a 14″ Sony CRT portable TV because of this effect.