Slide 87 of the pdf has the best summary, but the main thing w/r/r REST is that get/put of entire resources can be too heavy.
1. custom methods let you get/put specific fields, without having to transfer the whole thing or split resources up as you add new methods.
2. custom methods avoid needing to implement complex or heavy state mutations on the client. the example was rotating an image 90 degrees; you wouldn't want to download an entire full-res image to your wimpy battery-powered phone, rotate it, and re-upload it when you can just do that on the server.
(... of course this should just update some orientation metadata instead of resampling the image -- it's just an example)
1. custom methods let you get/put specific fields, without having to transfer the whole thing or split resources up as you add new methods.
2. custom methods avoid needing to implement complex or heavy state mutations on the client. the example was rotating an image 90 degrees; you wouldn't want to download an entire full-res image to your wimpy battery-powered phone, rotate it, and re-upload it when you can just do that on the server.
(... of course this should just update some orientation metadata instead of resampling the image -- it's just an example)