The ability to select different troop formations and different gear presets in the attack menu would be very useful and save alot of time. I'm not sure why this isn't already a thing.
I don't think it will really save any time since you can just change your gear and formation after your army starts marching. It may save you some time in changing to travel speed gear if you're trying to launch an attack very quickly while you're out of gear for one reason or another, but all in all, most people wouldn't benefit from this at all.
You can do it with 2 clicks Now.
You can form Your Gear Sets using Quick Swap and You can Form You battalion in Battle hall to save troops an heroes line up for the attack.
So basically this thing already exist separately.