I'm not entirely sure what's going on in the loops, but break will simply terminate that for-loop and jump back to the first one. So this is a performance gain since isPrime is set to false the first time you get into that if-statement. You don't need to continue that for-loop. So you will gain all those checks against the if-statement that are left to do once isPrime is false.

I might be wrong though ^^

I'm not entirely sure what's going on in the loops, but break will simply terminate that for-loop and jump back to the first one. So this is a performance gain since isPrime is set to false the first time you get into that for-loop. You don't need to continue that for-loop.

I might be wrong though ^^