Why is 5.c3 the most common move in the Ruy Lopez, Berlin defence, Beverwijk variation? Disclaimer: I'm a very, very green player.

After the first 4 moves, all of which seem natural given my understanding of basic opening principles (control the center, develop your knights, etc.), I notice 5. c3 is the most common move, according to this database:

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1. e4 e5
2. Nf3 Nc6
3. Bb5 Bc5
4. O-O Nf6
5. c3


I'm not sure why, though. In particular, it seems really passive. Is it for an eventual d4? If not, what do you see as the reason for this being so common? I would think, e.g., 5. Nc3 would be more common.
I'm not really familiar with this variation, but c3 is a typical move in the Ruy Lopez for a couple of reasons: one, which you already mentioned, is to support d4 (and in this variation, d4 has the added benefit of forcing the black bishop to retreat). The other is to open c2 as a retreat for the bishop, which has a tendency to get pushed back by pawns and then threatened with being traded for a knight. In the Ruy Lopez, the b1 knight is often developed via d2.