I use an e-file at prep and at infill when I remove the old product, but I finish file with a hand file because there's rarely much to file after I've applied the product. But when removing old product at infill I use an e-file, but this can really depend on your average customer too. A lot of my customers love encapsulated designs whether it's glitter or otherwise, and therefore removing it with an electric file is better for my soul and my shoulders.
And if you use coloured acrylic like Ciderella mentioned, then you definitely want an e-file.
The efile is also great for filing underneath the nail to cut in the C-Curve after several infills when it would normally start to disappear, and also for filing underneath to fix nails that grow downward. Also if you need to remove the nail underneath the nail to reduce lifting from free edge, it's perfect for that, but you have to have the correct bits and training for all efile situations.
The benefits of efile in those situations is to reduce the strain on your body. And doing infills every day is where the real benefit of an e-file comes in.
If I was doing only new sets, then I wouldn't need an e-file at all. But I never do new sets, only infills, so an e-file is invaluable.