There are certainly tasks that are best done by a phone or mobile app; usually, these are things that involve moving around, such as navigation, or depend on phone sensors, such as working out which way is "up". But nearly everything else can be done by a website.
I'd hate to be a one-man developer trying to maintain a cross-platform mobile app. I sympathise. But strictly from the sidelines; I long ago decided that I wasn't interested in playing games with proprietary platforms and gatekeepers.