Most fraction representations work well for adding fractions with like denominators. Fraction bars are particularly good for adding if the sum is greater than 1. The numberline format makes it clear that we are using the whole to measure with, and that it makes sense to have a fraction greater than one:
In this fraction bars example students can compare the sum to 1 to convince themselves that 4/5 + 3/4 = 7/5 = 1 2/5:
Whereas with this fraction squares problem, you will often encounter the question: does this show 5/4 or 5/8?
This confusion is often persistent, and comes from students limited understanding of fractions in a part-whole model. I have even read one suggestion that young children should work with fractions in a ratio context before moving to a part-whole model.