Substrate is an effect device to give your bass sounds the right “boost”. It will generate additional harmonics to the sound in the low end frequency range (sub-harmonics) based on a pre-set fundamental frequency from which it will generate the harmonics. These harmonics can be added and amplified by a “boost level” which results in a bass sound with more power in the low end range.

The second advantage is, if the frequency is set exactly to the fundamental frequency of a sound, the generated harmonics will produce a “psycho-acoustic effect” that gives your brain the feelings to perceive the sub frequency range of a bass sound on weak systems that can’t reproduce such low frequencies (like weak laptop speakers or mobile devices), even if these low frequencies are not really audible.

Resource: https://mastrcode-music.de/en/vst-plugins/substrate/