Every 2 weeks I plan out a menu for Matt and me. It helps me to have things planned out so I'm not trying to figure out every single night what to fix for dinner. It also makes it easier to plan out my grocery shopping. It's cheaper too! Because I know exactly what I'm cooking in advance, I know precisely what I need from the market!
I plan in for leftovers, and I'm also flexible on what days I make things. For example: last week I had leftovers planned for Wednesday's dinner. We ended up unexpectedly feeding the missionaries that night, so I switched Wednesday and Thursday, and fed the Elder's crepes for dinner.
For us, planning for leftovers is so important! I have a hard time just cooking for two, and if I didn't plan in leftover nights, there would be a lot of food that ended up in the trash...and THAT is not a good idea!