Why do we use tf.name_scope()

They are not the same thing.

import tensorflow as tf
c1 = tf.constant(42)
with tf.name_scope('s1'):
    c2 = tf.constant(42)
print(c1.name)
print(c2.name)

prints

Const:0
s1/Const:0

So as the name suggests, the scope functions create a scope for the names of the ops you create inside. This has an effect on how you refer to tensors, on reuse, on how the graph shows in TensorBoard and so on.