Why study the opening? Note I say the opening, not a specific line. Learn plans, not openings. Which means familiarising yourself from as many typical middlegames arising from as many openings as possible. In other words, look at and try to understand as many openings as possible.
Legend has it that no less than Viswanathan Anand attributes a degree of his abilities to a personal revelation in understanding concerning how the many different openings relate. To know what is better or worse about one position from one opening as compared to a similar position from another opening gives us an indicator as to how to play the position in front of us. The more positions you know, the stronger a player you become.
Here's a young Magnus Carlsen on the wrong end of Francisco Vallejo's understanding of chess. Note that Carlsen is playing the Sicilian, but, in the opening phase, he gives his opponent the option of transposing to either a Caro-Kann or French-type position.