It makes the car:
Quieter. With a significant reduction in wind noise at high speed.
Less likely to be broken in to.
A better place to be when it is hot. A better place to be when it is cold.
Stiffens the chassis to improve general handling.
Gives you the option of turning your cabriolet into a coupe.
And subjectively, some would say it makes the car better looking.
Reasons not for owning a hardtop:
You spend 90% of the time driving with the top down.
Basically, having a hardtop gives you options.
You've got a Cayman when you want one and a Boxster when you don't.
jh