Coaxial speakers do not NEED crossovers, but they will perform better if they have them. Crossovers are electronic filters that divide the signal going to a speaker into different frequency bands, and send those bands to the appropriate speaker driver. This is necessary because there is usually a big difference in how well different drivers can reproduce different frequencies. For example, tweeters are great at reproducing high frequencies, while woofers are great at reproducing low frequencies. By sending only the appropriate frequencies to each driver, crossovers help to ensure that all the drivers in a coaxial speaker are working together to produce accurate sound.