You are correct. The end result is the same if you create big $ single credit vs having small $ multiple credits. In your example, the choice is to either combine them now or later, with the same end result. I generally keep credits of similar value to what it would cost me/family to fly our typical city pair.

There are advantages in keeping multiple credits instead of combining them into big ones especially if you are going to transfer them. When you transfer a big credit, you loose out on the transferability of the dollar amount left over after paying for the fare. It can be mitigated, but you have to take extra steps to "regain" the transferability. In my family of 3, I fly the least and my teenage child flies the most between college and home. I am more comfortable in keeping the bulk of our credits in my account rather in my child's account and parcel out the credit when the need arises, purely for security reasons. Smaller credits make that goal easier for me in my use case.