MSPeconomist

For me, the plus/minus 3 days enables me to search a week at a time (without clicking back and forth from the calendar to see the details as no, I don't want some crazy routing with long layovers even if it is available for minimum miles) and more importantly to save those searches so that they can be redone with just a click. I'm comparing EF to delta.dumb here, although I do very much miss the ability to search for upgrade space on DL using EF.

EF is also a great way to see fare class availability, for example if you want to SDC from a coach ticket on DL, or when trying to construct your own ticket by picking individual segments that have the fare class you need.

With the latest DL "enhancement" to not show seat maps for other cabins unless you do a fresh dummy booking, it can be quick and easy to check seat maps (for example, as a very imperfect way to assess upgrade odds) on EF rather than delta.dumb.

With practice, you'll learn the things that EF does well and the things where you can get the needed information more efficiently from other sources.