People are all different. The idea of getting flowers for your partner, or leave cheesy notes for them, might come natural to some and not to others. Another comment pointed out how having ADHD makes it hard to remember to do the sort of stuff this app might suggest, so it can be a great help.
Besides, is this different from scrolling through social media and seeing couples activities and deciding to try them? Is this different from seeing a florist ad while walking and deciding to buy flowers? If anything, going out of your way to install an app shows more care and effort than these "spontaneous" activities.
At any rate, spontaneity is overrated, especially in relationships (maybe because of Hollywood relationships?). Constance, effort, care, are more important... you still need to keep things fresh tho