OK there are many types of effects but when it comes to mono/stereo think of three types. Mono = 1 input and 1 output. Mono to stereo 1 input and 2 outputs and true stereo 2 inputs 2 outputs.
In the world of hardware, if you record a mono source one mic or input you would use the EQ on your mixer and insert your compressors and other dynamic effects. All this would be done on one mono channel of your mixer as it is only one audio. Most stereo dynamic effects are just two mono ones with a link function.
If you put that single audio onto a stereo track which is two mono tracks with the aim of keeping it mono you are now having to process two identical audio streams with two matching effects (remember stereo eqs and compressors are two identical effects one working on the L channel and the other on the R. If what comes out of the 1st output does not exactly match that coming out of the 2nd your 2 mono signals will not be lined and could sound poor.
OK to recap on this bit. For dynamic effect processing on a mono source it is more reliable to use mono dynamic effects …keep it as one signal in and one out.
Most delays, chorus effects work by having a single input that after treatment comes out of two outputs one sent to the l and one to the right …It is the time/pitch difference from the input that makes the effect and the difference from one output to the other that makes the effect stereo.
Quote “A typical stereo delay has a mono input and a stereo output, although a delay plug-in can (and most likely will) have a stereo input as well. On a stereo delay, as you know, you can adjust the left and right delay lines individually, thus creating a stereo effect. I’d feed a mono signal into a stereo delay. If you head for a stereo delay effect, it will most likely sound more stereo and more focused if you feed the delay with a mono signal than if you feed it with a stereo signal”.
If you want to add an effect to a collective of mono signals for example all the individual tracks of a drum kit you best option is to route to a stereo group, pan individual tracks from the mono source and use your stereo effect across the stereo field of the group.
For creating a space a true stereo reverb maybe best. The most practical way is to create a stereo fx channel and send you individual mono sources to this buss. Remember if you sing in a room you are singing in mono but what you hear with the acoustics added is stereo.
Of course rules are there to be broken.
Mono = one audio source
Stereo = two audio sources. I can see no advantage in turning one audio source into two identical audio sources.