Here is what I've seen described as the main line of the Closed Ruy Lopez up to move 7:
[fen ""] 1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 Nf6 5. O-O Be7 (5... b5 6. Bb3 Be7 7. Re1 d6) 6. Re1 b5 7. Bb3 d6
I'm interested in Black's fifth move in particular. The top three options, with number of games according to the lichess opening book, are
- Be7 (43,520). Described in books as going to the Closed Ruy Lopez.
- b5 (8,126). Described as going to the Arkhangelsk variation.
- Nxe4 (4,844). Open Ruy Lopez.
Now 5...Be7 is clearly the most popular choice, but after 5... b5 6. Bb3, Black could choose to transpose to the Closed Ruy Lopez with 6... Be7 (shown as a variant in the diagram above). Here are the frequencies after 6. Bb3:
- Bb7 (3,452). Arkhangelsk variation.
- Bc5 (2,827). Neo-Arkhangelsk variation.
- Be7 (1,765). Transposing back to the Closed Ruy Lopez.
My question is why is 5...Be7 overwhelmingly more popular than 5...b5? Assuming Black wants to get into the Closed Ruy Lopez, what would they lose by playing 5...b5? An argument for the latter is that by playing 5...b5, Black doesn't need to worry about the Delayed Exchange variation.
Judging by the book, the transposition is doable and some people do play it, but they are a small minority (less than 5%).