Why? My guess is one part style and two parts specific performance reasons but just a guess.
First I'd say that's what one of the designers thought looked cool and distinctive as a styling feature but, not obviously everyone
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From a design handling point of view It does give the area on the hull leading into this feature a little more flare correctly known as "flam" which provides a drier ride as it deflects waves and spray down generated by the bow in a following sea better.
Areo dynamics on the faster hulls are helped as the shape of the hull this area allows makes a contour that generates a little lift from the air, can be seen pretty pronounced on the GT 150 hull.
My 2 cents.
