No, you don't need it in addition to a graphics program, but many choose to have both. An assembly, or layout program, is usually so fast and easy to use that being able to put something together quickly (either simple graphic style, or using a kit, etc) is really nice.
I use a lot of programs, and usually choose based on how complex I want to get. If it's an easy layout, I use an easy software for speed.
A nice combo is soemthing like DIP because it's super easy to use, yet has versatile text options, can have plugins, and can somewhat make some elements.
I use FotoFusion for the quick and dirty layouts, but I'm not very good at it yet.
What graphics software do you have? Welcome to DSP!
SELLING MY SIGMA NIKON F 10-20mm f/4-5.6 ULTRA WIDE-ANGLE LENS