I won't comment on the benifit of lats for boxing.
The muscles on the side of your body Latisimus dorsi, give you body the Wide V look
The best excersise for building big lat spread is seated rows
Sitting on your a** with legs straight out in front knees slightly bent
pulling on bottom pulley of a a cable machine with weight stack.
you have to tilt "forward " on your hips or a** bones and then curve your back so your belly is sticking out.
Form is VERY important in this excersise.
you can do a pull with palms up or down on a bar or together on a v HANDLE WITH THUMBS UP.
Pull to middle of chest or with palms up to your sides keep the hands high about mid chest.
keep elbows in
a small rocking is ok to move the weight.
start light, warm up. go big.
I used to do a weight stack and finish with sets of 12 at 700 or so pounds.
On smaller weight stacks you can do it with a guy standing on the stack
Along with bench press a huge excersise to build upper body power and size.
One of the basics
Bench,rows,curls,squat,deadlift.leg press.tricep pushdown