A user can have multiple files/folders in separate places to copy on a drive. If the user doesn't select them all at once to copy them, each new batch of copies will be started simultaneously with the already copying batch. If the destinations are on the same drive, this affects the operations' performance dramatically and, I think, can cause more fragmentation (when applicable).
Users could be offered a "queue file copy operations" option in dolphin settings, which would make a new batch of copy operations, wait before the previous one is finished before starting. Maybe, I don't know if it's possible, there could even be a "smart queuing" feature which would allow simultaneous operations if the destinations are all on different drives, or something.

Reproducible: Always

Steps to Reproduce:
1. Copy some (large) files from some folder to the other
2. Before the first operation is finished, start a new copy operation of more files.
3. Repeat
Actual Results:  
All copy operations run at the same time, resulting in very slow performance and possibly fragmentation.

Expected Results:  
If "Queue copy operations" option is enabled, all copy operations launched while there is an ongoing operation are queued and wait before the preceding operations are completed before starting.

All file managers I used (nautilus and its forks, windows explorer) do launch copies at the same time. If this is implemented, dolphin might be the first one to provide this feature.