your arm extend, turn and move under the skin... having staples avoid the expansion of the skin to extreme, which would cause rupture of the stitches...

for a 2 inches scar, no need, but more... it's like an horror show when the scar reopen just because you stretched your arm too far... lol

i can tell, i had a 20inches long scar on the chest 18 years ago, and they stapled me more than 50 times to fix it solid until is heal... lol