Not everyone uses the Julian calendar, or even 4-digit years. Here in Japan, this is year 4 of the Reiwa era, so you need a year picker that lets you choose the correct era (Reiwa, Showa, etc.) and then the year of that era, and of course the number of permissible years per era differs greatly. (The era reflects an emperor's term, so when we get a new emperor, we start a new era.) Then, just because this system is such a PITA and anything international doesn't use it (but everything government-related does), we need a converter that lets you convert between the Japanese standard and the Julian calendar.
I wouldn't say the Japanese calendar is used for "most" activity. As far as I can tell, the Japanese calendar is certainly used for things like government forms, but for non-government stuff, not so much. If I buy tickets for something online, for instance, the web form asks for my birthdate using the normal Gregorian calendar year. Even the government stuff is a mix; my residence card shows my birthdate and card expiration date with the Gregorian year.
Not everyone uses the Julian calendar, or even 4-digit years. Here in Japan, this is year 4 of the Reiwa era, so you need a year picker that lets you choose the correct era (Reiwa, Showa, etc.) and then the year of that era, and of course the number of permissible years per era differs greatly. (The era reflects an emperor's term, so when we get a new emperor, we start a new era.) Then, just because this system is such a PITA and anything international doesn't use it (but everything government-related does), we need a converter that lets you convert between the Japanese standard and the Julian calendar.