Why we can't build a To-Do app as a single tier application?

In this example - Educative: Interactive Courses for Software Developers

To-Do list app is given as a 2-tier service, but considering the performance, look and feel, it will be almost similar as UI and business logic reside in client’s computer then why we are storing DB in another computer, very less network calls can be a reason but 1-tier can send the numbers to 0 ?

Just wanted to know why 2-tier is favourable then 1-tier in this case ?

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@Ved_Sinha We can build a To-Do app as a Single-Tier application. There is no wrong or right answer to it but as mentioned there will be higher security threats. If we think about a simple to-do app, almost all the critical data is stored in DataBase(This could be a single point of failure or data loss for apps like To-Do. It is always better to provide separation of concerns but keeping in mind the latency and performance. ) which is hosted on a different machine in the case of Tier-2. We have added extra security compared to Tier-1. All in all, Tier-2 is a kind of balance between security and latency.
Hope it helps.