Is it useful to study openings you will never play?

For example:

I reply to e4 with e6 or e5.

As white, I never play e4. Almost exclusively D4. Nf3 rarely.

So is it still helpful to my improvement to study about the caro kann and Sicilian?

The move by move details probably not. The early middlegame themes, structures and typical manoeuvers certainly yes, as it improves general chess background.

no

It's useful to know openings that aren't your "main" opening, it teaches you different plans and patterns. There are also more opportunities for transpositions, or near transpositions, then is obvious. But you can't really know an opening without playing it some. Also just to choose a repertoire requires a broad understanding of the opening. How do you know your "main" opening is the right opening for you if you don't know any others?

Long story short, the opening theory? No! Similar to what post 2 said, yes.

As a player that plays 1...e6 and 1...e5, there is no need to learn Sicilian theory. However, exposure to annotated games with the Sicilian would be very useful for the basics of chess - Strategy, Tactics, etc.

Find a player from the past whose games you enjoy looking at. For me that is Korchnoi, Stein, Nezh, Botvinnik, Short, Gelfand, Yusupov, Anderson, etc. Go through a game collection of theirs. Go through ALL games, not just Frenches and 1...e5's.