Yes mostly to keep your OS Partition small for easy quick Images to restore if you screw up.
There is absolutely no reason if you want, to just merge the 2 partitions in to 1
Mini tool Partition Wizard is free and makes it easy to accomplish this. I would advise moving any data off the D partition before you Delete D Partition then expand C into the unused space.
G752VY-DH72 Win 10 Pro
512 GB M.2 Samsung 960 Pro
1 TB Samsung 850 pro 2.5 format
980m GTX 4 GB
32GB DDR 4 Standard RAM
Z97 PRO WiFi I7 4790K
Windows 10 Pro
Z97 -A
Windows 10 Pro