Are Sumps necessary?

I am in the very early stages of researching my very first saltwater tank.
I have a 30gal that I plan to house zoas, (2)clowns,(1) firefish goby, and (1)Tail Spot Blenny.

Would a sump be necessary for my plans? Or would a HOB be sufficient?

I don't understand all the ins and outs of sump workings so I would need an explanation if it is deemed necessary.

Thanks!:)