[Rainbow Six]

On a purely practical level they take up less space against the dock.

I'm in my office right now and am about thirty feet away from one of Aberdeen's harbour's quays. There are four offshore support vessels tied up alongside right now, all backed in (or ass end if you prefer). There's space for two more, so that piece of quay can accommodate six vessels.

If they were tied up alonsgide long ways the same space would accommodate at most two (and I'm not even sure about that - might just be one).

I imagine it's also substantially easier for them to come and go. When they're ready to leave all they have to do is go directly forward (not sure what the technical maritime term is) to get into the open water. When they come in they effectively reverse in, so no tugs or other forms of assistance required.