Why is this "Ruy-Lopez-CC-Sequence" so "Popular?"

Emilio, I see you; have only done two lessons and your ratings are very low.

If you are enthusiastic about chess, I suggest not getting caught up in studying openings in detail. Instead you should be working mostly on tactcis plus some basic opening and endgame theory and some basic openings where the path to the middlegame is clear: a chess master who lectured to the high school state-3rd-ranked chess club I sponsored answered the question of "What's your favorite opening?" with "Anything that gets me to a playable middle game.

WHERE do you expect to get playing the Black Side of the Ruy Lopez?  If you play the Caro-Kann or French Defense you would know, in many games, that you need to play ...c5 at some point and launch a counter attack on the Q-side.  If you play the Bishop's Opening, you know you want to play f4 before playing Nf3 and launch a K-side attack.  Look for openings that show you what to do.

As far as tactics go, working tactics trainers here and at places like chesstempo.com are great for building tactical skills, especially if you review every puzzle and ask yourself: 1) which tactics were used, 2) why did it take me so long to see it (or miss it).

If you don't know the names of the tactics or can't state the differences and similarities between the Dovetail Mate and the Swallows Tail Mate or an X-Ray Attack and a Skewer, you've playing the game with partly empty toolbox.

I suggest taking some time and learning these tactics to the point where you can eventually demonstrate each of the them - knowing them by name allows for quicker mental recall:

https://www.chess.com/article/view/chess-tactics--definitions-and-examples

https://chesstempo.com/tactical-motifs.html

As Martin Weteschnik says in his very excellent Chess Tactics from Scratch, 2nd Ed. (c.2012) (pp. 16-23):  "Solving tactical puzzles without fully understanding the underlying mechanisms is not the most efficient way to learn. Instead you must first understand the elements of combinations....[the patterns] might look...trivial...but might turn up in complicated situations. Only if you know these simple patterns by heart will you be able to recognize them in very difficult situations. Have you ever lost due to an unforeseen [tactic]? [From such] games,...put the positions on a board and try to figure out why these [tactics] came as surprises.  Don't be satisfied with just being able to pinpoint the exact mistakes. Always try to understand the underlying causes of your defeats. In some sense all defeats are caused by lack of understanding. So the question one must ask oneself after a loss is: What more do I need to understand to improve my chess?

Good luck!

I was once your rating, focus on tactics and puzzles rather than openings, for a 1200 player ,your opening understanding shouldn't be derived from memorizing opening lines ...

Is there a reason to suppose 1618_0f_PsyChess is focusing on openings and attempting to derive opening understanding from memorizing opening lines? Indeed, it seems to me that the question suggests something quite different - an attempt to understand the popularity of 1 e4 e5 2 Nf3 Nc6 3 Bb5 a6, rather than simply memorize it.