One additional note: The /hotbar command works on the straight hotbars mainly used by keyboard/mouse players. There's a corresponding /crosshotbar or /chotbar command for the cross hotbars that controller players use. Copy works just the same. Set is functionally the same, but instead of a number for the bar and a number for the slot on that bar, you have a number for the bar followed by a name for the slot.
For instance, the crossbar slot on the left side activated by L2 + the down directional button can be called LDD (for left trigger - direction buttons - down) or LD4, whereas the slot on the right side activated by R2 and the triangle button is called RAU (right trigger - action buttons - up) or RA2.
>>Slot Names:
L2+← LDL (LD1)
R2+← RDL (RD1)
L2+↑ LDU (LD2)
R2+↑ RDU (RD2)
L2+→ LDR (LD3)
R2+→ RDR (RD3)
L2+↓ LDD (LD4)
R2+↓ RDD (RD4)
L2+□ LAL (LA1)
R2+□ RAL (RA1)
L2+△ LAU (LA2)
R2+△ RAU (RA2)
L2+X LAR (LA3)
R2+X RAR (RA3)
L2+〇 LAD (LA4)
R2+〇 RAD (RA4)
It's for when you have more skills than you can fit on your most conveniently available hotbars.
He gave a couple of examples. You might want one hotbar layout when facing lots of enemies, one that places your AoE skills most prominently, and another for when you're facing one or two enemies in which your single-target skills are more prominent. Or as healer might want one while in Cleric Stance and another while in regular healing stance.
He showed at the start how to copy your bars to somewhere to store them, but putting those back works just the same. With your AoE layout for instance stored on ACN bars 1 and 2 and your single-target ones stored on ACN 3 and 4, you'd then have a "load single target layout" macro that copies ACN 3 and 4 back onto your main SMN 1 and 2 bars, and a "load AoE layout" macro that copies ACN 1 and 2 onto your main SMN 1 and 2 bars. During battle as Summoner, you could swap your hotbars around as needed by using one or another of those layout loading macros. You'd be able to stay on the Summoner 1 and 2 bars throughout the duty while having different sets of abilities available there.
If you just have a couple different layouts like in this example, its easy to set up a toggle between them. That "load single target layout" macro should then be included on one of the slots in the AoE bars, while that same slot in the single-target bars has your "load AoE layout" macro. That key could then always toggle your layout back and forth when you need it.