I think I know what Actions does in Photoshop, but I never generally used it. I know someone who automated a work process of applying filters and effects using it, but I never touched it.
What's your favourite workflow using Actions?
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Sign up to join this communityBatching. You can Batch with many options, for example - converting a series of 100 images all to low res jpg, or resizing (as one answer already states) to "Auto-levelling" each image.
Actions allow you to record a repetitive task and repeat by pressing a button, instead of doing it all over again.
Resizing. If I have fifty full-size photos which need to be made 240px wide and 72 dpi, doing a Batch with a preset Action takes about a minute.
I use actions for simple but redundant things. Like placing a file, copying and pasting layer files or even applying very specific filter settings.