What would be something you can achieve using ML that you couldn't do with the stock firmware and postprocessing?

The list is so long. My favorite is the internal intervalometer + ETTR. Canon has always been laughed at for not having an internal intervalometer, and ML proves how lame it is to not have one. ETTR (Expose To The Right) is an auto metering mode that allows the camera to keep the histogram pushed as far to the right (better exposure) automagically by increasing shutter time and/or increasing ISO. This is essential for doing holy grail timelapse of sunset/sunrise where the exposure is constantly changing. This feature alone is worth it's weight in gold.

However, a lot of the features exposed are more video oriented. The Canon bodies were primarily photo cameras that could shoot video in a cumbersome way. ML brings features a video shooter would need without diving into the menus like audio metering. The older bodies also have hardware limitations on write speed, so people use the HDMI out to external recorders to record a larger framesize/bitrate/codec than natively possible. Also, that feed normally has the camera UI overlay which prevents clean recordings. ML allows turning that off.

There are just too many features that ML unlocks. You'd really just need to find the camera body you are interested in using on their site, and see what it does for that body. Different bodies have different features. So some effort is required on your part to know exactly what it can do for you.

I don't know if modern cameras are better for this, but a big one historically was getting a clean, realtime HDMI output so that high quality cameras can be used with a capture card for broadcast purposes as a replacement for a webcam. Manufacturers understand that that's a "pro" level need/feature and have intentionally segmented the market so that lower-tier devices can't do it even though the hardware is obviously all present.

- Lua script support. It is not complete (in ML hardly anything is) but allows to access a lot of ML and Canon functions. Years ago someone made a script for automating solar eclipse shooting catching all critical phases while chilling and enjoying the view. - Introduced full electronic shutter (Silent Pic) for Digic 4 and 5. - Focus stacking for macro and - via Lua script - for landscape. - Exposure simulation switch for "cheaper" cams - Trap focus - Dual-ISO. Some HDR mode but without ghosting by manipulating sensor lines to record at different ISOs - Ghost image overlay - Customizable cropmark overlays (grids and others) - Fps finetuning. Several folks used it to record vintage monitors with very, very strange timings and without rolling bars.30.01 fps? No problem! - Zebras and focus peaking, vectorscope, wavelength monitoring, false colour support - RAW histogram - Bracketing with up to 11 frames (But why? ;-> ) - Intervalometer and bracketing (a bit more configurable than Canon has now) - Trigger by LCD's IR sensor (if any) or Audio (clap your hand) or motion detect - Rack focus - Display mirroring and upside/down options - Configurable presets (up to 15) - 30 minutes override for RAW recording, USB and HDMI streaming. Oh, and we have a new option to record native H.264/MOV for more than 29:59. Prototype but working. -Better AF micoradjustment for the cams having that option by Canon. - ,,,

Frankly: I once tried to maintain a help file and browsed through a lot of lesser known features. Took me days and I didn't even test RAW/MLV recording.

The big one for me was always focus peaking when using vintage lenses or doing IR photography. The extended White Balance settings were nice to have for IR, as well.