The case for going alone
Going to the cinema alone is one of the easiest solo activities in a city. You choose the time, choose the seat, arrive when you want and never compromise on the film. Once the lights go down, everyone is watching the same screen anyway.
For plenty of films, that is exactly the experience you want. Costar should never turn solo cinema into a problem that needs fixing.
When company changes the night
Other films make you want to talk. A divisive ending, a director you both follow, a restored classic or simply a Friday-night release can feel different when there is somebody beside you who deliberately chose it too.
The value is not that a stranger makes the film possible. You could already go. The value is that the same outing can also create a new connection.
Why a shared screening is useful
Meeting a new person can be awkward when the meeting itself is the whole event. A screening gives both people an independent reason to be there. That reduces pressure: the evening still makes sense even if the conversation is brief.
It also creates a natural boundary. You meet in a public cinema, watch the film and decide afterwards whether you want to keep talking.
Use both options
Keep going alone when that is what you want. Use Costar when there is a screening you would genuinely attend and you would enjoy the possibility of meeting somebody new there.
Those are complementary behaviours, not opposing identities.