First off, you should know that even if a case comes with a screen protector you can always install a different one (if the case allows it). There are some advantages to plastic variants, but glass screen protectors are better. One of the main reasons why is the toughness that tempered glass provides. They are much sturdier and are less prone to scratches.
Also, modern-age smartphones are adapting their design and adding more glass. For these types of phones, there is a noticeable difference when you install a plastic screen protector. In terms of how smoothly your finger glides, it may feel very rough. With a tempered glass protector, it will mirror the image of the smartphone itself and offer a smooth gliding motion.