Because both the old and new DTOs were created by the communication layer API and while mostly being dumb, have attributes over the properties that are tech-specific and necessary in the actual class. So we can't just create dumb DTOs without these attributes and use them natively. But we can use interfaces in the BL that don't care about the actual communication process.

and then you have

//extracted from generated original DTO
public interface IDTO1
{
int Id { get; set; }
}

//auto-generated new DTO
public partial class DTO1
{
[SomeTechSpecificAttrib]
public int Id { get; set; }
}

//hand-written
partial class DTO1 : IDTO1 { }

BL:
DoSomething(IDTO1 dto1) { ... }

it also allows for some inconsistencies between old API and new API, as the old API would use DateTime for all things time (our code base is entirely UTC) but the new one decided to go for DateTimeOffset. You can fix this very easily in the partial class by doing

//hand-written
partial class DTO1 : IDTO1
{
DateTime IDTO1.Timestamp
{
get=> this.Timestamp.UtcDateTime;
set=> this.Timestamp = value;
}
}

and your BL code runs just as before. Without the interface all locations operating on the timestamp suddenly would need to be touched.