If I understand your question correctly, then you're asking for the difference between inserts and sends. Inserts would be for processors that will work on your entire signal (i.e. compression, EQ...). Send effects are most commonly used for adding effects to a signal without changing the original signal. Reverbs are commonly used as send effects to make the signal seem closer or farther (for example) but without changing the actual signal.
In addition, send effects can be applied to multiple channels simultaneously, which can save a lot of processor power, as compared to inserting one on every channel that you want to use the effect on. This is especially true with reverbs, which tend to be processor hogs.
I second Mike's comment, and especially when you're doing things out of the box in the analogue domain: you've only got a limited number of FX units to use. You need to be able to add multiple signals into the same FX unit rather than using another FX unit on every channel that wants a bit of reverb.
So my advice is always: transfer the same "analogue" engineering principles into the box.... that always keeps me on track.