English opening, what is the usual purpose of Nb1 to Nc3 to Nd5?

I see this happen often in the English. Why does white do this? Is the idea to retreat back to Nc3, wait for black to play Nf6 to trade, or is there another reason?

There are several I have seen over the years, but here is one. Please don't read too much into the specific line because the focus of the question revolves around the placement of the N on d5 and not the actual line itself. That is why I didn't want to post a line to begin with. I am trying to find a general rule or pattern that can be identified in other lines as well.

Sure, it's 11 months later, but I have become interested in early Nd5 openings, so some thoughts are:

An early Nd5 is an aggressive incursion into Black's space, attacking four squares on Black's side while also covering four squares on White's side.

Attempts by Black to drive away the Knight will probably cause problems for Black:

a) Black c6 to attack the Knight puts another pawn on the long light-square diagonal (a8 to h1) -- too bad if Black was planning to fianchetto his c8 Bishop! The c6 pawn would also block the b8 Knight.

b) In general, Black Nf6 to attack the Knight could lead to NxN, then Black would have to choose between recapturing with the g7 pawn (not so good if Black planned to castle on that side), or take with the Queen (if not blocked by the e pawn), which exposes her early and might lose a tempo if Black needs to move the Queen again to get her out of danger.

c) Or, in games where White had already played e4, the Knight could retreat to e3 and would still have good coverage of the center.

d) In the specific diagram above, of course any attack on White's Knight should probably be answered with NxB (on b4).

In general we could say that an early Nd5 could give White a space advantage and could be disruptive to Black's opening plans. A disadvantage is that White is moving the same piece at least twice in the opening instead of developing something else.