Infantry is used to hold ground and form lines. Each of these stacks have 3-5 infantry support. The infantry also act as "soaks", that is, they absorb the brunt of the enemy attack, protecting the more valuable units.

With 15 artillery and repeated bombardment, this city has been reduced to rubble. Destroying the Barracks will prevent the enemy units from automatically replenishing their strength each turn. 


Note that the infantry has blocked all roads into the city to keep reinforcements out.

Actually destroying the roads is useful against a very powerful defense. Destroying the roads will slow the enemy's movement, cut off luxuries into the city possibly sending it into disruption, and cut off resources so the city cannot create advanced units.