Why do i need DesignTimeDbContextFactory

Hello

I have created an angular/asp.net core mvc application last weeks.
Everything was working fine.

I have added this line in startup.cs:

public void ConfigureServices(IServiceCollection services)
{
    services.AddDbContext<BddContext>(options => options.UseMySQL("Server=localhost;Database=testspa;Uid=root;Pwd=123456;"));

But today, i needed to create a new project, i get this error:

"Unable to create an object of type 'BddContext'. Add an implementation of 'IDesignTimeDbContextFactory<BddContext>' to the project, or see https://go.microsoft.com/fwlink/?linkid=851728 for additional patterns supported at design time."

I needed to remove the services.AddDbContext and i needed to create this file to make it works !

public class DesignTimeDbContextFactory : IDesignTimeDbContextFactory<BddContext>
    {
public BddContext CreateDbContext(string[] args)
        {
var builder = new DbContextOptionsBuilder<BddContext>();
builder.UseMySQL("Server=localhost;Database=testspa;Uid=root;Pwd=123456;"); 
return new BddContext(builder.Options);
        }
    }
}

It now work fine but my question is why should i need this file while it was working great before ?

Thanks

aa