It depends on the application, not all need a baffle step correction filter. But baffle difraction takes often a few db's from the bass frequency, and the BSC filter brings everything back in balance. Some technical explenation is found here: Understanding Baffle Step and Diffraction
But the filter is not mandatory, and some drivers (Mark Audio Alpair drivers as example) have a build in bsc in the frequency graph. Or you can also conpensate that in driver choice and crossover in a multway setup. it all depends on a lot of factors if it's needed or not. I use Mark Audio drivers in most of my builds and never really needed it.
And if done passive, it's mostly and inductor and a resistor, that create a lowpassfilter that reduce the output above the baffle step frequency a few dB's to bring all back in balance. This reduces the sensivity of the driver a few dB. When done active it's a low shelf eq that boosts the lower side of the output to compensate for the loss
But the filter is not mandatory, and some drivers (Mark Audio Alpair drivers as example) have a build in bsc in the frequency graph. Or you can also conpensate that in driver choice and crossover in a multway setup. it all depends on a lot of factors if it's needed or not. I use Mark Audio drivers in most of my builds and never really needed it.
And if done passive, it's mostly and inductor and a resistor, that create a lowpassfilter that reduce the output above the baffle step frequency a few dB's to bring all back in balance. This reduces the sensivity of the driver a few dB. When done active it's a low shelf eq that boosts the lower side of the output to compensate for the loss
There's a nice simple explanation of baffle step here Baffle Step Compensation and why it may or may not be necessary. Full range speakers, on the whole, lack bass, so they are more likely to benefit from some compensation than multiway speakers.