We would use a send and aux return when we want to use the same thing on multiple tracks at the same time, reverb for instance.
In the old days (pre-daw) a studio would often have 1 reverb unit (be it a digital one, or maybe a spare room that can be used... Motown records had an attic that was used for all their old songs... and if you work with a real old time recording guy or girl, they call it echo, not reverb). They would send any instruments to the room using the buss on their console, and the level being sent would alter the effect: louder reverb with less direct sound makes a sound move back into the sound field.
No, today, most daws have plenty of built in reverb effects (Logic has 6 I believe), some use less cpu power and others more. There was a time when we had to choose which reverb we would use depending on the computer: more powerful systems could use the Platinum Reverb, slower systems (laptops for example) would use the silver. Today we have reverbs that use a "tone picture" called an impulse response, and take allot of power to create.
Because of the limited availability of power, one would use 1 send ot buss to the aux, where the reverb would be located, allowing them to add reverb to whichever tracks they wanted too, and conserving power. Rather than use 1 reverb on each track (that can use a huge amount of power using an IR with a long decay time) we use 1 to create a cohesive sound in our mix.
I hope that makes sense. It's based on limitations which we really don't have today, especially with things like the UAD systems that use an extra card or 4 to add power to our systems, or multicore CPU's, or cheap outboard boxes (or expensive ones like the Bracasti M7, a stereo reverb that uses 10 of the same shark DSP chips that a UAD card uses to make the soundfield up, and sound unbelievable, or the old Lexicon 960 reverbs, that cost thousand each).
I crack up when I hear people go off about current DAW technology and systems that we have, remembering that 32 years ago I started recording with a Tascam 144 cassette 4 track recorder, and graduated to a Fostex G16 track 1/2 inch tape deck, a 6 ft rack of synths as well as a couple of controllers, 3 reverbs, and a Soundtrax PC midi board, 56 input with midi muting. I can do 10 times that on my Mac Book Pro, and about 20 on my custom build hackintosh.
I have been so lucky... the year I started recording as a pro midi started, as did computers and technology, and I went from 4 track tape to 2 track digital (the Digidesign Soundtools system) to 8 channels of Protools 1, to Logic and my audio interface (I have about 6 of them, so it depends on what I'm doing as to which I use). It's been an incredible journey. I'd suggest you take some time to check out some books on what it was like (Computer Music has a 20th anniversary issue out now that comes with issue 2 as a bonus, and it's so interesting to see where this stuff came from).
Apple, Atari, Commodore, these were the first computers, Tascam, Fostex, Otari, these guys made the first affordable tape decks, Roland, Korg, Ensoniq, Emu, Sequential Circuits, Korg, to name a few, brought us the synths and instruments we needed to do our jobs and record, Smpte Tracks, Opcode Vision, Mark Of The Unicorn, and C-Lab Notator, allowed us to record and edit midi data. Then we for Propellerheads, the creators of the first system to move data from a virtual instrument into a daw, and that changed everything.
I mention these guys because some are gone (Opcode were the real motivators behind standard midi files, Roland, Yamaha, and Sequential were the guys who worked on the Midi spec that makes when we do today possible), and most people don't think that Propellerheads should be honored with some kind of lifetime award for the things they have come up with that transformed our ability to create.
Anyways, sorry for the rant. it's been a fantastic journey, and I love it when people actually give a damn enough to learn about this. The "craft" of audio engineering is actually being lost. Most people have no idea how to mic a drum kit, or setup a session for a band or an orchestra.I just hope that some of the real craft of what we do gets lost to time and technology.
I can tell you this: Recording with Barry Manilow, at Capitol Records here in LA, with Allen Sides recording, and Michael Lloyd producing, with a 40 + piece orchestra live off the floor, was a moment I will never forget as long as I live. And I hope that some of you get to go that far too, the people that dig, ask questions, respectfully be a PITA, but also dig out their own to discover what it is we can really do, who find the magic, and have moments in the studio that become highlights on their lives.
Merry Xmas LUG... I'm happy we're all here, and I'm honored to be part of this unique community. Thanks...