You present a counterpoint. While some people enjoy using it to pay for dinners and houses, others see in Bitcoin a way to make profit, selling it more expensive than they bought. If people are using BTC for these reasons, it's because they need it on their individuality, and fiat wasn't fulfilling their expectations and personal needs.
It's hard to say why we need Bitcoin in the modern world because each person has their own personal reasons. But on general it's like asking why do we need money in the modern world... Bitcoin is just money, with some extra advantages decentralization and technology give us.
Because this will make our world modernize by using online systems through financing and money transfer. This will be convenient and more reliable to like the banking system. This will also serve as a medium of banking and money exchange. As what I always stating in the forum this will be more convenient for hotel booking, flights, reservations and paying through the online system.