The analogue soundtrack on 35mm is recorded optically and also used as a fallback in case the digital soundtrack fails for some reason. It often used Dolby Stereo which could be played as a regular stereo track but also as four channels including centre and surround.

70mm film was far superior with 6 discrete magnetic soundtracks. But it was also far more expensive and required entirely new equipment.

Digital soundtracks were a solution to retrofit discrete multi-channel audio to existing 35mm technology. I love how they found space next to and between the perforations. It reminds me of how colour TV works. DTS is one that used the timecodes and came on an external CD which would also be far cheaper and something cinemas could easily upgrade to without replacing existing equipment.

The LFE channel appeared with these formats and is another example of supporting retrofitting. The idea with LFE is cinemas could simply buy and install some subwoofers and feed the LFE channel directly to them. No expensive bass processing required. No need to touch the existing main channels. Easy upgrade. Now every cinema has bass management so it's not really necessary but it's still there.