I'm working on one of Silberjahr-Ländler for an outdoor wedding and was reminded why I started to mark lever changes in highlighter on some of my music.
When I was new to levers, it was mostly so I wouldn't forget the lever change. As comfort grew with lever changes, I stopped marking so many, and when playing from lead sheets, don't usually mark them at all.
But, there's more than one reason to highlight your lever changes. In some music, especially music you don't play often, it's hard to keep your bearings. Eyes flitting back and forth between music and levers makes it difficult to keep one's place.
The other reason to mark levers with highlighter is to help your eyes get back to the proper measure the music after finding the lever. When I'm playing outdoors, where there is more visual distraction, this habit has been very helpful.