Of course it matters and it's a feature which DL advertises heavily.

If you can't get it to work online or on the app, call DL. It helps to check the menu first, as phone agents might not read the full description of the choices to you.

I try to look for the email confirmation as well as in the app for proof of what I've ordered (and to remind me), although I realize that it's not really proof as I could have changed or cancelled my order later. I'm not sure I always get the email, so I'll print a hard copy if I can do so easily. The fact that you've preordered should also show on the boarding pass, but not the choice you made--and by check in, you can't fix anything if it's within T-24. Of course, nothing really helps if your meal choice isn't loaded when the aircraft is catered.