Tactics question: units with varied equipment

Hello 40k folks! So I'm building up a shiny new unit of Nobz right now (non-Mega). Pretty much every guide/review I've read on the unit suggests getting as much variation between individual members of the squad as possible. This leads to my query:

I vaguely understand why this is beneficial; wound allocation and such. But I'm still having trouble fully wrapping my head around the concept, as it's been a long time since I actually played the game. Just wondering if anyone can explain the concept more fully, and maybe give a few tips for such a unit (who should be fielded in the front, etc.).

For reference: the unit is going to be two klaws (one with a bosspole), two big choppas (one with a bosspole), a single kombi-scorcha, a banner, and the last four may just be choppa/sluggas.