It should be noted that some dry/wet knobs play games with how the ratios work and actually make the 50/50 spot = 100/100, in which case it's acting more like a parallel processing knob. But generally speaking, they are a simple dry/wet ratio. If you dropped a verb on your kick drum channel as an insert and cranked up the Wet, your kick is going to effectively disappear into the verb. If you just Send to a verb from that kick, your kick will remain solid (no loss of level) while adding in the verb from the send (added, technically, to the master mix, not back to the original channel).
All this assumes we are talking Post-Fader on any Sends. When you switch to Pre-Fader you can actually replicate the other half of what the dry/wet knob does -- e.g. dropping the volume of the original channel while increasing the amount of added effected.
I'm not trying to makes things overly complex here, I just think it's funny how many discussions of Insert Vs. Send pop up across audio forums and virtually nobody discusses the inherent differences between Serial and Parallel processing. To me, this is the most key factor in deciding whether I want to use an Insert or a Send. It's not so simple as just "Sends exist so you can have multiple instruments going to one effect!" though certainly that's a part of it. "
also you could consider the following article but bear in mind this relates to effects pedals, but the principle and theory is applicable to plugins also