Every car audio systems needs speakers if you want to hear the music, but the type can drastically change the overall sound. If you just want to replace a bad speaker any drop in replacement will work and their quality does not really differ by that much. To make a real difference you will want to use amplification and step up to components separates and they truly add a distinctive “voice’ to your sound.
Picking component separates are sort of like picking a fine wine. While there are standards of excellence, there still is personal choice and an acquired understanding of how the music should be presented. In the audio industry we call the difference in sound from original “color.” In other words, if the sound is warmer or brighter than the original sound we call that coloration - it is key to the difference in the type of components that you buy and will make the overall voice of your system. No single purchase will make more difference in how things sound than component speakers, so if it is possible to listen to them before you buy, you should. If you can’t listen to the speakers it is OK, you just need to define the type of music you want and you can go from there. If you want music to sound the exact way it was recorded that is how I rate speakers.
In my top five I rated the speakers based first upon their overall lack of coloration and then based upon how accurate they would behave through the range of volume listening levels from low to high, it is a term called linearity.