Do I have to use a roll crimp?

[long colt lawrence]

I've reloaded a great deal for cowboy action needs but still pretty green at shotgun reloading. Can anyone tell me, aside from immediate identification of the round, why a roll crimp is needed for reloading sabot slugs as opposed to a the standard crimp. I've got BP sabots and plan to start working with them. Any advice or lessons learned would be appreciated.  Thanks.

[eye shot]

Star crimp is just fine on some slugs. It takes more of the shell up than a roll crimp.