Effect of logical fragmentation of range scan in SQL Server Table
Can someone tell me why? Why is the physical order important at all?

Please note, I am speculating as to why the decisions were made given the best possible information I have available to me.
AFAIK it's due to the optimization of IO using a specific API which in this case is called ReadFileScatter.
ReadFileScatter requires a starting offset and a number of bytes to read from that point. Once read the number of contiguous on disk bytes are transferred into non-contiguous memory locations (aka buffers).
Because Read-Ahead, depending on version and edition, can read different amounts of data it ends up being a single large IO instead of a bunch of tiny ones which makes the throughput of the overall storage system more albeit at a cost to latency.
/Start of pure speculation/
Please keep in mind that when these APIs and choices were being made, rotational media (aka spinning rust) was the main source of storage and SSDs did not exist. Thus, having all of the items contiguous on disk means we don't have to reposition the track heads between reads which makes the overall single contiguous read faster than many smaller disjointed ones.
References and further reading:

Reading Pages (Books Online)
Sequential Read Ahead by Craig Freedman
How It Works: Bob Dorr’s SQL Server I/O Presentation
What do I need to know about SQL Server database engine I/O?

The first reference includes a diagram you may find helpful: