Yes, you can use component speakers without an amp, directly connected to the head unit. However, an amplifier provides better power delivery and performance, improving sound quality.
If your car features either factory speakers or aftermarket ones, or has recently received upgrades that add new ones, an amplifier should be used to boost them. Most stock car stereo units do not produce more than 15-18 watts RMS per channel and may cause your speakers to distort and sound poor if pushed too hard; adding an amp is the best way to boost their performance and will result in superior sounding speakers than those installed by default.
An amplifier provides better audio clarity and cleaner soundstage than stock stereo units or factory-installed component speakers, drawing audio enthusiasts towards adding one in their vehicle. A good amp should feature enough power rating to not only meet speaker wattage needs but also accommodate subwoofer usage; additionally it should offer multiple inputs so you can connect to CD players, radios or aux-in inputs easily. Finally it’s essential that an amp has an excellent frequency response that spans across its operating frequency range for optimal playback experience.
Frequency response of speakers determines their ability to accurately reproduce both low and high parts of the frequency spectrum. Component speakers tend to produce full-range sounds with higher frequencies and wider range than coaxial sets due to having independent components tuned individually for specific frequencies, creating greater tone clarity and soundstage than with coaxial sets.
Component speaker sets include both tweeter and woofer drivers, but most also contain a crossover that divides audio signals among them, with highs typically handled by the tweeter, mids/lows handled by the woofer, and everything in between handled by another driver (usually tweeter for highs and woofer for mids/lows/mid-range/lows). This gives your system an improved soundstage; further improvement can be accomplished with proper placement of each speaker in your vehicle. Although installing new component speakers is more complex than doing coaxial setup, installing new component speakers may pay dividends thanks to superior audio quality offered.