Ancillary proceedings usually take place when the decedent lived in one county, and you are dealing with real property in another county.
So, in an example, the decedent dies in County A, and probate is opened there. Decedent owns real property in County B, but there are many issues associated with this property causing delays.
The estate attorney may open probate in County A promptly, but wait until the issues with the real property are straightened out somewhat before filing an ancillary proceeding in County B. There isn't a requirement to wait, but the primary probate (in County A) will give the estate authority to manage the decedent's affairs, and minimizing the amount of work done in County B's courts may be worthwhile.
This is simply a strategy consideration that the probate attorney may be taking into consideration.
(Here is a very good primer on probate in South Carolina: http://barristerstitle.com/barristersforms/Estates-Training-Session.pdf).
But again, as noted, the above is simply speculation, if you are uncertain as to why an attorney is conducting business in a particular fashion, you can always ask them, most are happy to explain (assuming you are not an adverse party).